General Concepts
Clinical Manifestations
The genera Escherichia, Klebsiella, Enterobacter, Serratia, andCitrobacter (collectively called the coliform bacilli) andProteus include overt and opportunistic pathogensresponsible for a wide range of infections. Many species are members of thenormal intestinal flora. Escherichia coli (Ecoli) is the most commonly isolated organism in the clinicallaboratory.
Enteric Infections: E coli is a major enteric pathogen, particularly indeveloping countries. The principal groups of this organism responsible forenteric disease include the classical enteropathogenic serotypes (EPEC),enterotoxigenic (ETEC), enteroinvasive (EIEC), enterohemorrhagic (EHEC), andenteroggregative (EAEC) strains which are described in detail in Chapter 25.
Nosocomial Infections: Coliform and Proteus bacilli currently cause 29 percentof nosocomial (hospital-acquired) infections in the United States. In order ofdecreasing frequency, the major sites of nosocomial infection are the urinarytract, surgical sites, bloodstream, and pneumonias. This group of nosocomialpathogens are responsible for 46% of urinary tract and 24% of surgical siteinfections, 17% of the bacteremias, and 30% of the pneumonias. Ecoli is the premier nosocomial pathogen.
Community-Acquired Infections: E coli is the major cause of urinary tract infections,including prostatitis and pyelonephritis; Proteus,Klebsiella,and Enterobacter species are also commonurinary tract pathogens. Proteus mirabilis is the most frequentcause of infection-related kidney stones. Klebsiella pneumoniaecauses a severe pneumonia; K rhinoscleromatis causesrhinoscleroma; and K ozaenae is associated with ozena, anatrophic disease of the nasal mucosa.
Structure, Classification, and Antigenic Types
The coliforms and Proteus are Gram negative bacilli. All generaexcept Klebsiella are flagellated. Some strains producecapsules. Virulence often depends on the presence of attachment pili (which canbe characterized by specific hemagglutinating reactions). Sex pili also may bepresent. The major classes of antigens used in defining strains are H(flagellar), O (somatic), and K (capsular).
Pathogenesis
E coli enteropathogens have diverse mechanisms for diseaseproduction which include different toxins and colonization factors (see Ch. 25 ). Specific serotypes ofcoliforms and Proteus with particular virulence factors often preferentiallyinfect specific extraintestinal sites. E coli bacilli inextraintestinal infections have soluble and cell-bound hemolysins, siderophores,capsules, and adherence pili.
Host Defenses
Coliforms and Proteus species rarely cause extraintestinaldisease unless host defenses are compromised. Disruption of the normalintestinal flora by antibiotic therapy may allow resistant nosocomial strains tocolonize or overgrow. The skin and mucosae may be breached by disease, trauma,operation, venous catheterization, tracheal intubation, etc. Immunosuppressivetherapy also increases the risk of infection.
Epidemiology
The epidemiology of coliform and Proteus infections involvesmany reservoirs and modes of transmission. The infecting organism may beendogenous or exogenous. Transmission may be direct or indirect; vehiclesinclude hospital food and equipment, intravenous solutions, and the hands ofhospital personnel. Nosocomial strains progressively colonize the intestine andpharynx with increasing length of hospital stay, resulting in an increased riskof infection.
Diagnosis
The clinical picture depends on the site of infection; diagnosis relies onculturing the organism and on biochemical and/or serologic identification. Avariety of phenotypic (i.e., biotyping, serotyping, antibiograms, bacteriocinand phage typing) and genotypic (i.e., plasmid analysis, RFLP, ribotyping, andPCR) methods are used for epidemiological investigations.
Control
The most effective way to reduce transmission of nosocomial organisms is for allhospital personnel to wash hands meticulously after attending to each patient.Vaccines and hyperimmune sera are not currently available. Various antibioticsare the backbone of treatment; drug resistance (often multiple) due toconjugative plasmids is a major problem.
Introduction
The Gram-negative bacilli of the genera Escherichia, Klebsiella,Enterobacter, Serratia, Citrobacter,and Proteus(Table 26- 1) are members of the normalintestinal flora of humans and animals and may be isolated from a variety ofenvironmental sources. With the exception of Proteus, they aresometimes collectively referred to as the coliform bacilli because of sharedproperties, particularly the ability of most species to ferment the sugarlactose.
Table 26-1
Taxonomy of Selected Coliform Bacilli and Proteus in Human ClinicalSpecimens.
Many of these microorganisms used to be dismissed as harmless commensals. Today, theyare known to be responsible for major health problems worldwide. A limited number ofspecies, including E coli, K pneumoniae, Enterobacter aerogenes,Enterobacter cloacae, S marcescens,and P mirabilis,are responsible for most infections produced by this group of organisms. Theincreasing incidence of the coliforms, Proteus, and otherGram-negative organisms in diseases reflects in part a better understanding of theirpathogenic potential but more importantly the changing ecology of bacterial disease.The widespread and often indiscriminate use of antibiotics has createddrug-resistant Gram-negative bacilli that readily acquire multiple resistancethrough transmission of drug resistance plasmids (R factors). Also, development ofnew surgical procedures, health support technology, and therapeutic regimens hasprovided new portals of entry and compromised many host defenses.
Clinical Manifestations
As opportunistic pathogens, the coliforms and Proteus take advantage of weakened hostdefenses to colonize and elicit a variety of disease states (Fig. 26-1). Together, the many disease syndromes produced bythese organisms are among the most common infections in humans requiring medicalintervention.
Figure 26-1
Sites of colonization and extraintestinal disease production by thecoliforms and Proteus.
Enteric Infections
The role of E coli as a major enteric pathogen, particularly indeveloping countries, is discussed in detail in Ch. 25.However, the different types of Ecoli associated with enteric infections and which are classifiedinto five groups according to their virulence properties are briefly describedhere: Enteropathogenic (EPEC) serotypes in the past were associated with seriousoutbreaks of diarrhea in newborn nurseries in the US. They remain an importantcause of acute infantile diarrhea in developing countries. Disease is rare inadults. Enteroinvasive (EIEC) types produce disease resembling shigellosis inadults and children. Enterotoxigenic (ETEC) types are a major cause oftraveler's diarrhea, and of infantile diarrhea in developing countries.Enterohemorrhagic E. coli(EHEC) occur largely as a singleserotype (O157:H7) causing sporadic cases and outbreaks of hemorrhagic colitischaracterized by bloody diarrhea. EHEC also may cause hemolytic uremic syndrome(HUS), an association of hemolytic anemia, thrombocytopenia, and acute renalfailure. Enteroaggregative (EAEC) types exhibit a characteristic aggregativepattern of adherence and produce persistent gastroenteritis and diarrhea ininfants and children in developing countries.
Nosocomial Infections
The etiology of nosocomial infections has markedly changed during past decades.Streptococci were the major nosocomial pathogens in the preantibiotic era.However, following the introduction and use of sulfonamides and penicillin,Staphylococcus aureus became the predominant pathogen inthe 1950's. Aerobic gram negative rods gained prominence as nosocomial pathogenswith widespread use of aminoglycosides and first generation cephalosporinsthrough the early 1970's. Subsequent widespread use of broad spectrumcephalosporins was associated with changes in the frequency and etiology ofnosocomial infections into the 1980's with the trend towards certaingram-positive pathogens. For example, in nosocomial bloodstream infections from1980 to 1989 marked increases in the incidence of coagulase-negativestaphylococci, S. aureus, enterococci, and Candidaalbicans infections occurred.
The coliforms and Proteus were responsible for 29 percent ofnosocomial (hospital-acquired) infections in the United States from 1990 through1992 based on data from hospitals participating in the National NosocomialInfections Survey (NNIS) (Table 26-2).Estimates of nosocomial infections in US hospitals suggest that about 5 percentof the estimated 40 million annual admissions, or 2 million patients, had atleast one nosocomial infection. Thus, the coliforms and Proteusprobably are responsible for hospital-acquired infections in approximately600,000 patients each year. Aside from the enormous cost measured in human life,nosocomial infections prolong the duration of hospitalization by an average of 4days and increase the cost of medical care by $4.5 billion a year in 1992dollars.
Table 26-2
Frequency of Selected Pathogens Causing Nosocomial Infectionsa.
The highest numbers of nosocomial infections in the NNIS occur in surgical andmedicine services. Among surgical patients, highest rates of nosocomialinfections occur with surgery on the stomach (21%) and bowel (19%), craniotomies(18%), coronary artery bypass graft procedures (11%) and other cardiac surgery(10%). High rates also are observed with burn (15%) and high-risk nurserypatients (14%). In order of decreasing frequency, the major sites of nosocomialinfection are the urinary tract, surgical sites, bloodstream, and lowerrespiratory tracts. The coliforms and Proteus were responsiblefor 46% of urinary tract and 24% of surgical site infections, 17% of thebacteremias, and 30% of the pneumonias from 1990 through 1992.Escherichia coli, the predominant nosocomial pathogen, isthe major cause of infection in the urinary tract and is common in other bodysites. Staphylococcus aureus and Pseudomonasaeruginosa are currently the most common pathogens in nosocomialpneumonias, followed by Enterobacter andKlebsiella. Coagulase-negative staphylococci have replacedE coli as the predominant pathogen in primary bloodstreaminfections. The major causes of surgical site infections are Saureus, coagulase- negative staphylococci, andenterococci.
Other coliform bacilli and Proteus have been incriminated invarious hospital-acquired infections. Klebsiella,Enterobacter,and Serratia species are frequent causesof bacteremia at some medical centers and also are frequently involved ininfections associated with respiratory tract manipulations, such as tracheostomyand procedures using contaminated inhalation therapy equipment.Klebsiella and Serratia species commonlycause infections following intravenous and urinary catheterization andinfections complicating burns. Proteus species frequently causenosocomial infections of the urinary tract, surgical wounds, and lowerrespiratory tract. Less frequently, Proteus species causebacteremia, most often in elderly patients. A series of nationwide outbreaks ofbacteremia (1970 to 1971 and 1973), caused by contaminated commercial fluids forintravenous injections, involved Enterobacter cloacae, Enterobacteragglomerans,and C freundii.
The role of Citrobacter species in human disease is not as greatas that of the other coliforms and Proteus. Citrobacterfreundii and C diversus (C koseri) have beenisolated predominantly as superinfecting agents from urinary and respiratorytract infections. Citrobacter septicemia may occur in patientswith multiple predisposing factors; Citrobacter species alsocause meningitis, septicemia, and pulmonary infections in neonates and youngchildren. Neonatal meningitis produced by C diversus, whileuncommon, is associated with a very high frequency of brain abscesses, death,and mental retardation in survivors. Although E coli and groupB streptococci cause most cases of neonatal meningitis, the most common cause ofbrain abscesses in neonatal meningitis is P mirabilis.
Immunocompromised patients often develop non-hospital-acquired infections withcoliforms. For example, group B streptococci and E coli areresponsible for most cases of neonatal meningitis, with the latter accountingfor about 40 percent of cases. Infections seen in cancer patients with solidtumors or malignant blood diseases frequently are caused by E coli,Klebsiella, Serratia,and Enterobacter species.Such infections often have lethal course. Individuals who are immunosuppressedby therapy (e.g., cancer patients or transplant recipients) or by congenitaldefects of the immune system may develop Klebsiella,Enterobacter, and Serratia infections. Manyadditional factors such as diabetes, trauma, and chronic lung disease maypredispose to infection by coliforms and other microbes.
Community-Acquired Infections
The coliform organisms and Proteus species are major causes ofdiseases acquired outside the hospital; many of these diseases eventuallyrequire hospitalization. Escherichia coli causes approximately85 percent of cases of urethrocystitis (infection of the urethra and bladder),about 80 percent of cases of chronic bacterial prostatitis, and up to 90 percentof cases of acute pyelonephritis (inflammation of the renal pelvis andparenchyma). Approximately one half of females have had a urinary tractinfection by their late twenties due to E coli from their fecalflora. Proteus, Klebsiella, and Enterobacterspecies are among the other organisms most frequently involved inurinary tract infections. Proteus, particularly Pmirabilis, is believed to be the most common cause ofinfection-related kidney stones, one of the most serious complications ofunresolved or recurrent bacteriuria.
Klebsiella was first recognized clinically as an agent ofpneumonia. Klebsiella pneumoniae accounts for a smallpercentage of pneumonia cases; however, extensive damage produced by theorganism results in high case fatality rates (up to 90 percent in untreatedpatients). Klebsiella rhinoscleromatis is the agent ofrhinoscleroma, a chronic destructive granulomatous disease of the respiratorytract that is endemic in Eastern Europe and Central America. Klebsiellaozaenae, a rare cause of serious infection, is classicallyassociated only with ozena, an atrophy of nasal mucosal membranes with amucopurulent discharge that tends to dry into crusts; however, recent studiesindicate that the organism may cause various other diseases including infectionsof the urinary tract, soft tissue, middle ear, and blood.
Distinctive Properties
Structure and Antigens
The generalized structure and antigenic composition of coliform bacilli, as wellas of Proteus and other members of the familyEnterobacteriaceae, are depicted schematically in Figure 26-2. A more detailed figure of thestructure is presented in Chapter2. The major antigens of coliforms are referred to as H, K, and Oantigens. The coliforms and Proteus are divided into serotypeson the basis of combinations of these antigens; different serotypes may havedifferent virulence properties or may preferentially colonize and producedisease in particular body habitats. The H antigen determinants are flagellarproteins. Escherichia coli, Enterobacter, Serratia,Citrobacter,and Proteus organisms are peritrichous(i.e., they have flagella that grow from many places on the cell surface).Klebsiella species are nonmotile and nonflagellated andthus have no H antigens.
Figure 26-2
Structure and antigenic composition of coliforms andProteus species.
Some strains of coliform and Proteus species have pili(fimbriae). Pili are associated with adhesive properties and, in some cases, arecorrelated with virulence. Different pilial colonization factors generally aredetectable as hemagglutinins that can be distinguished by the type oferythrocyte agglutinated and by the susceptibility of the hemagglutination toinhibition by the sugar mannose. Sex pili, which have receptors for‘male’ specific bacterial viruses and are geneticallydetermined by extrachromosomal plasmids, are important in coliform ecology andin the epidemiology of diseases produced by coliforms andProteus species in that sex pili are involved in genetictransfer by conjugation (e.g., chromosome-mediated and plasmid-mediated drugresistances or virulence factors).
Major Surface Antigens
K antigens (capsule antigens) are components of the polysaccharide capsules.Certain K antigens (e.g., K88 and K99 of E coli) are pilus-likeproteins. The K antigens often block agglutination by specific O antisera. Inthe past, K antigens routinely were differentiated into A, L, and B groups onthe basis of differences in their lability to heat; however, these criteria aresubject to difficulties that make the distinction tenuous. SomeCitrobacter serotypes produce Vi (virulence) antigen, a Kantigen also found in Salmonella typhi. Species ofProteus, Enterobacter,and Serratiaapparently have no regular K antigens. However, the K antigens are important inthe pathogenesis of some coliforms. A diffuse slime layer of variable thickness(the M antigen) also may be produced but, unlike the K antigens, it isnonspecific and is serologically cross-reactive among different organisms.
The outer membrane of the bacterial cell wall of these species contains receptorsfor bacterial viruses and bacteriocins (plasmid-encoded, antibiotic likebactericidal proteins called colicins in E coli that are activeagainst the same or closely related species). The outer membrane also containslipopolysaccharide (LPS), of which the lipid A portion is endotoxic and the O(somatic) antigen is serotype specific. The serologic specificity of the Oantigens is based on differences in sugar components, their linkages, and thepresence or absence of substituted acetyl groups. Loss of the O antigen bymutation results in a smooth-to-rough transformation, which often involveschanges in colony type and saline agglutination, as well as loss of virulence .Certain strains of P vulgaris (OX-19, OX-2, and OX-K) produce Oantigens that are shared by some rickettsiae. These Proteus strains are used inan agglutination test (the Weil-Felix test) for serum antibodies producedagainst rickettsiae of the typhus and spotted fever groups (see Ch. 38).
Toxins
Enterotoxigenic strains of Klebsiella, Enterobacter,Serratia,Citrobacter, and Proteus also have beenisolated from infants and children with acute gastroenteritis. The enterotoxinsof at least some of these organisms are of the heat-labile and heat-stable typesand have other properties in common with the E coli toxins (seeCh. 25). However, theimportance of the coliforms and Proteus, other than E coli, inenteric infections is questionable
Pathogenesis
The process of disease production by coliforms is, in many cases, poorly understood.Production of disease by coliforms or Proteus species inextraintestinal sites often involves specific serotypes of the organisms and specialvirulence factors. For example, respiratory tract infections by Kpneumoniae predominantly involve capsular types 1 and 2, whereasurinary tract infections often involve types 8, 9, 10, and 24. Similarly, only a fewpolysaccharide K antigens (types 1, 2, 3, 5, 12, and 13) of E coliare found with high frequency in urinary tract and other extraintestinal infections.These observations suggest that different serotypes may have specificpathogenicities. An alternative explanation is that such strains may simply be themost prevalent types in the normal gut flora.
There is good evidence for specific pathogenicity in E coli strainsthat cause extraintestinal infections (Table26-3). Approximately 80 percent of E coli isolatesinvolved in neonatal meningitis carry the K1 antigen, a fact attributable, at leastin part, to the higher resistance to phagocytosis of K1-positive strains. Certain Oantigens (O7 and O18) are found in combination with K1, usually in strains that areisolated from cases of neonatal bacteremia and meningitis and that show increasedresistance to the bactericidal effects of serum complement. Interestingly, theE coli K1 antigen, composed of neuraminic acid, shows immunecross-reactivity with the group B meningococcal polysaccharide capsule.
Table 26-3
Virulence Factors of E coli Isolates fromExtraintestinal Infections.
Escherichia coli strains isolated from extraintestinal infectionsoften possess a number of properties not usually found in random fecal isolates.These include production of soluble and cell-bound hemolysins, the colicin Vplasmid, production of the siderophores aerobactin and enterochelin, and specialpilial antigens for adherence to target cells. The hemolysin kills host cells andmakes iron more available by releasing hemoglobin-bound iron from lysed red cells.To strip iron from the host iron-binding proteins ( transferrin and lactoferrin),E coli produces siderophores of both the hydroxamate(aerobactin) and phenolate (enterochelin) types. Common or type 1 pili may mediateadherence to bladder cells; P-pili are virulence factors for strains causingpyelonephritis; S-pili, which recognize O-linked sialo-oligosaccharides ofglycophorin A, are associated with meningitis and urinary tract infections. Certainafimbrial adhesions and outer membrane proteins also have been associated withurinary tract infections.
The enzyme urease, produced by Proteus, and to a lesser extent byKlebsiella species, is thought to play a major role in theproduction of infection-induced urinary stones. Urease hydrolyzes urea to ammoniaand carbon dioxide. Alkalinization of the urine by ammonia can cause magnesiumphosphate and calcium phosphate to become supersaturated and crystallize out ofsolution to form, respectively, struvite and apatite stones. Bacteria within thestones may be refractory to antimicrobial therapy. Large stones may interfere withrenal function. The ammonia produced by urease activity may also damage thepithelium of the urinary tract.
Except in cases of bacteremia and other systemic infection, there is little evidencethat endotoxin plays a role in most coliform and Proteus diseases.Humans with coliform bacteremia show many of the typical effects of endotoxin,including fever, depletion of complement, release of inflammatory mediators, lacticacidosis, hypotension, vital organ hypoperfusion, irreversible shock, and death.
Host Defenses
It cannot be overemphasized that coliforms (except for E coli inenteric diseases) and Proteus species are unlikely to cause diseaseunless the local or generalized host defenses fail in some way. The normalgastrointestinal flora, which includes E coli and, frequently,other coliforms and Proteus species in small numbers, is importantin preventing disease through bacterial competition. Prolonged antibiotic therapycompromises this defense mechanism by reducing susceptible components of the normalflora, permitting nosocomial coliform strains or other bacteria to colonize orovergrow.
The organisms may breach anatomic barriers through third-degree burns, ulcersassociated with solid tumors of the skin and mucous membranes, intravenouscatheters, and surgical or instrumental procedures on the biliary, gastrointestinal,and genitourinary tracts. The lungs may be violated by instrumentation, as intracheal intubation, or even by aerosols from contaminated nebulizers orhumidifiers, which carry organisms to the terminal alveoli.
Corticosteroid administration, radiotherapy, and the increased steroid levelsassociated with pregnancy tend to decrease host control over infections (e.g., bydepressing the immune response). Cytotoxic drugs also are immunosuppressive.Cancer-or drug-induced neutropenia is an important predisposing factor inbacteremia. Devitalized tissue or foreign bodies may be a source of organisms andmay also shelter the organisms from phagocytes and antimicrobial factors.
The interaction of multiple predisposing factors often determines the clinical courseand outcome of coliform or Proteus infection. For example, themortality of bacteremia increases progressively when the underlying disease (e.g.,cancer or diabetes) is rated as nonfatal, ultimately fatal (death within 5 years),or rapidly fatal (death within 1 year). Similarly, coliform and Proteus infectionscommonly are more severe in the very old and very young.
Epidemiology
The epidemiology of coliform and Proteus infections is complex andinvolves multiple reservoirs and modes of transmission. Klebsiella,Enterobacter, Serratia, Citrobacter, and Proteusspecies live in water, soil, and occasionally food and, in many cases, form part ofthe intestinal flora of humans and animals. Escherichia coli isbelieved not to be free living, and its presence in environmental samples is takenas indicating recent fecal contamination. In fact, water quality is determined bythe presence of the rapid lactose fermenting E coli, Klebsiella,and Enterobacter (coliform counts ) and Ecoli(fecal coliform counts) using special selective media.
Coliform and Proteus organisms causing infection may be exogenous orendogenous. While most nosocomial infections appear to arise from endogenous flora,studies of hospitalized adults and infants have shown that the intestinal tract isprogressively colonized by nosocomial coliforms with increasing length ofhospitalization. Patients being treated with antibiotics, severely ill patients, and(probably) infants are more likely to be colonized, and other sites of colonizationsuch as the nose and throat may be important in such patients. Colonized patientshave a higher risk of nosocomial infection than patients who are not colonized.
The bacteria may be acquired indirectly via various vehicles or by direct contact . Avariety of vehicles have been implicated in the spread of nosocomial pathogens. Forexample, Klebsiella, Enterobacter, and Serratiaspecies have all been recovered in large numbers from hospital food, particularlysalads, with the hospital kitchen being a primary source . An outbreak of urinarytract infections due to multiply drug-resistant S marcescens wasassociated with contaminated urine- measuring containers and urinometers. Seriousoutbreaks or individual cases of bacteremia due to coliforms have been associatedwith extrinsic contamination of intravenous fluids or caps during manufacture andwith extrinsic contamination of intravenous fluids and administration sets in thehospital environment. Other medical devices and medications have served as vehiclesfor the spread of nosocomial pathogens. Occasionally, transmission may be viamembers of the hospital staff who are colonized with nosocomial pathogens in therectum or vagin* or on the hands; however, passive carriage on the hands of medicalpersonnel constitutes the major mode of transmission.
Certain properties of the coliforms may be important in the epidemiology ofhospital-acquired infections. Coliform bacteria other than E colifrequently are found in tap water or even distilled or deionized water. They maypersist or actively multiply in water associated with respiratory therapy orhemodialysis equipment. Klebsiella, Enterobacter, andSerratia species, like Pseudomonas species,may exhibit increased resistance to antiseptics and disinfectants. The same group ofcoliforms has a selective ability over other common nosocomial pathogens (includingE coli, Proteus species, Pseudomonas aeruginosa,andstaphylococci) to proliferate rapidly at room temperature incommercial parenteral fluids containing glucose.
Diagnosis
Because the coliforms and Proteus can cause many types of infection,the clinical symptoms rarely permit a diagnosis. Culturing and laboratoryidentification are usually required. Selected characteristics that are useful in thedifferentiation of coliform bacilla and Proteus species found inhuman clinical specimens are shown in Table26-4. The organisms have simple nutritional requirements and grow well onmildly selective media commonly used for members of theEnterobacteriaceae, but not on some moderately and highlyselective enteric plating media (Salmonella-Shigella, bismuthsulfite, and brilliant green agar). Extraintestinal specimens such as urine,purulent material from wounds or abscesses, sputum, and sediment from cerebrospinalfluid should be plated for isolation on blood agar and a differential medium such asMacConkey or eosin-methylene blue agar. The finding of more than 105organisms/ml in clean voided midstream urine is often taken as‘significant bacteriuria.’ However, in acutely symptomaticfemales and with other types of specimens (i.e., those obtained by catheterizationor suprapubic aspiration) from either sex, a more appropriate threshold,particularly in the presence of pus cells and the absence of epithelial cells, mightbe more than 102 colonies of a known uropathogen/ml. Because urine is agood growth medium for many microbes, specimens should be refrigerated (4°C)if transport to the laboratory is delayed longer than 30 minutes, unless a urinetransport container with preservative is used.
Table 26-4
Differentiation of Coliform Bacilli and Proteus Found inHuman Clinical Specimens.
Isolation of certain coliforms or Proteus species from fecalspecimens may be facilitated by adding a moderately selective medium such asxylose-lysine-desoxycholate (XLD) or Hektoen enteric agar. Use of tetrathionate orselenite broth for enrichment of enterotoxigenic strains from feces is notrecommended because both media inhibit various genera of coliforms. The strong(E coli, K pneumoniae, Enterobacter aerogenes) and occasionallythe slow or weak (Serratia, Citrobacter) lactose-fermentingcoliforms produce characteristic pigmented colonies on the enteric plating media. Astriking characteristic of Proteus species is their propensity toswarm over the surface of most plating media, making the isolation of otherorganisms in mixed cultures difficult. The swarming growth appears as a rapidlyspreading thin film, sometimes with changing patterns of whirls and bands. SorbitolMacConkey agar is useful for screening EHEC (commonly E. coliO157:H7) on which sorbitol-negative colonies are nonpigmented and consideredsuspicious for the organism. Unless the physician specifically requests that thelaboratory look for the possibility of E coli as an enteropathogen,tests for pathogenic strains, including toxin assays, serotyping, and serogrouping,will not be done.
In cases of suspected bacteremia, replicate bottles (one cultured aerobically, theother anaerobically) containing 25 to 100 ml of appropriate medium withanticoagulant (e.g., sodium polyanetholesulfonate) are inoculated with 10-mlportions of blood. It is usually necessary to take multiple specimens, both beforeand after antibiotic therapy is started. It is important to take specimens afterantibiotic treatment is started so that therapeutic failure can be recognized whilethe bacteremia may still be amendable to more aggressive medical or surgicaltreatment.
All of the coliforms and Proteus species are Gram negative,facultative anaerobic, non-spore- forming rods that are typically motile, except forKlebsiella, which is nonmotile. The oxidase test is negative,and nitrates are reduced to nitrites. Proteus species and allcoliforms ferment glucose, but fermentation of other carbohydrates varies. Lactoseusually is fermented rapidly by Escherichia, Klebsiella and someEnterobacter species and more slowly by Citrobacter and someSerratia species. Proteus, unlike thecoliforms, deaminates phenylalanine to phenylpyruvic acid, and it does not fermentlactose. Typically, Proteus is rapidly urease positive. Somespecies of Klebsiella, Enterobacter, and Serratiaproduces a positive urease reaction, but they do so more slowly. A battery of testsfor biochemical properties is required to identify the coliforms andProteus to the species level. Commercial identification systemsare now widely used by most US clinical laboratories and consist of‘kits’ or miniaturized biochemical tests which are read manually(e.g., API-20E and BBL Crystal) or automatically (e.g., Vitek or MicroSCAN).
The coliforms are characterized by great antigenic diversity caused by variouscombinations of specific H, K, and O antigens. For example, approximately 50 H, 90K, and 160 O antigens have been identified among various strains of Ecoli. In contrast, Klebsiella, with no H antigens, has10 O antigens and approximately 80 K antigens. Serologic identification of thecoliforms and Proteus species, commonly by reference laboratories,is an extremely important epidemiologic tool. Similarly, other phenotying methodsincluding biotyping (biochemical profiles), antibiograms (patterns of resistance toantimicrobal agents), and bacteriocin and phage typing have been widely used inepidemiologic studies, particularly of multiresistant isolates of coliforms andProteus. Recently, genotyping methods such as plasmid profiles (determined byagarose gel electrophoresis), RFLP (restriction fragment link polymorphism) of totalDNA, pulsed-field gel electrophoresis, targeted analysis of DNA polymorphism,ribotype, and arbitrarily primed PCR (polymerase chain reaction) have been used inepidemiological studies. In hospital-acquired infections, for example, the same or asmall number of serologic or plasmid types suggests single sources of infection. Thefinding of multiple serotypes or plasmid profiles suggests multiple sources ofinfection or endogenous infections.
Control
Prevention of coliform and Proteus infections, particularly thosethat are hospital acquired, is difficult and perhaps impossible. Sewage treatment,water purification, proper hygiene, and other control methods for enteric pathogenswill reduce the incidence of E coli enteropathogens. However, thesecontrol measures are rarely available in less developed regions of the world.Breast-feeding is an effective means of limiting outbreaks of enteropahogens ininfants. Aggressive infection control committees in hospitals can do much to reducenosocomial infections through identification and control of predisposing factors,education and training of hospital personnel, and limited microbial surveillance.Except for investigations of potential outbreaks, routine culturing of personnel,patients, and the environment is not warranted. Selective decontamination of thedigestive tract with a suitable nonabsorbable antimicrobial regimen may be usefulduring outbreaks caused by nosocomial coliforms and Proteus.Meticulous hand washing after each patient contact a highly effective means ofreducing the transmission of nosocomial pathogens (Fig. 26-3)is infrequently or poorly performed by some hospitalpersonnel. In a study conducted in an intensive care unit following an educationalcampaign on the importance of hand washing, the compliance was 17 percent forphysicians, 100 percent for nurses, 82 percent for respiratory technicians, and 88percent for diagnostic services personnel.
Figure 26-3
Major routes of transmission and prevention of spread of nosocomialpathogens.
Active or passive immunization against coliforms and Proteus species is notpracticed. However, vaccines or hyperimmune sera for the six common Gram negativepathogens (E coli, Klebsiella, Enterobacter, Serratia, Pseudomonasaeruginosa,and Proteus) probably would have a majorimpact on morbidity and mortality from nosocomial infections. In a trial, themortality was reduced markedly in a group of patients with Gram-negative bacteremiawho had been given antiserum against a mutant E coli with anexposed lipopolysaccharide core.
Ampicillin, sulfonamides, cephalosporins, tetracycline,trimethoprim-sulfamethoxazole, nalidixic acid, ciprotloxacin, and nitrofurantoinhave been useful in treating urinary tract infections by coliforms and Proteusspecies. Gentamicin, amikacin, tobramycin, ticarcillin/clavulate, imipenem,aztreonam, and a variety of third-generation cephalosporins may be effective forsystemic infections; however, laboratory tests for drug susceptibility areessential. For example, resistence of E coli to ampicillin, andfirst generation cephalosporins is increasing rapidly to the extent that they can nolonger be considered primary drugs of choice in empirical treatment of urinary tractinfections. Likewise, emergence of coliforms with chromosomal or plasmid-encodedextended spectrum B-lactamase activity is causing global problems with resistance tothird generation cephalosporins. Some coliforms have multiple resistance due to thepresence of R plasmids transmissible by conjugation. Conjugative resistance plasmidsallow the transfer of resistance genes among species and genera that normally do notexchange chromosomal DNA (Ch. 5). Insome cases, resolution of the infection may require drainage of abscesses or othersurgical intervention.
Measures commonly used to control epidemics of antibiotic resistant Gram-negativebacilli have included: (1) closing the unit to new admissions until control of theoutbreak is underway; (2) reinforcing hand-washing practices; (3) gown and gloveisolation, often combined with isolation of patients in separate quarters; and (4)restricting the use of the antibiotic to which the offending clone is resistant.
References
Beck-Sague C, Villarino E, Giuliano D. et al. Infectious diseases and death among nursing home residents:results of surveillance in 13 nursing homes. Infect Control Hosp Epidemiol. 1994;15:494. [PubMed: 7963443]
Bergeron MG. Treatment of pyelonephritis in adults. Med Clin North Amer. 1995;79:619. [PubMed: 7752732]
Bingen, E: Applications of molecular methods toepidemiologic investigations of nosocomial infections in a pediatric hospital.Infect Control Hosp Epidemiol 15, 1994 . [PubMed: 7963442]
Bodey, GP, Elting LS, Rodriguez S, Hernandez M. Klebsiella bacteremia: a 10-year review in a cancerinstitution. Cancer. 1989;64:2368. [PubMed: 2804929]
Brun-Buisson C, Legrand, P Can topical and nonabsorbable antimicrobials prevent crosstransmission of resistant strains in ICUs? Infect Control Hosp Epdemiol. 1994;15:447. [PubMed: 7963436]
Conley JM, Hill S, Ross J. et al. Handwashing practices in an intensive care unit: the effects ofan educational program and its relationship to infectionrates. Am J Infect Control. 1989;17:330. [PubMed: 2596730]
Emori GT, Gaynes RP. An overview of nosocomial infections, including the role of themicrobiology laboratory. Clin Microbiol Rev. 1993;6:428. [PMC free article: PMC358296] [PubMed: 8269394]
Foxman, B, Zhang L, Palin K. et al. Bacterial virulence characteristics of Escherichiacoli isolates from first-time urinary tractinfection. Infect Dis. 1995;171:1514. [PubMed: 7769286]
Gordon MC, Hankins GDV. Urinary tract infections and pregnancy. Comp Ther. 1989;15:52. [PubMed: 2676334]
Johnson JR, Stamm WE. Urinary tract infections in women: diagnosis andtreatment. Ann Intern Med. 1989;111:906. [PubMed: 2683922]
Horan TC, Culver DH. et al. Nosocomial infections in surgical patients in the United States,January 1986-June 1992. Infect Control Hosp Epidemiol. 1993;14:73. [PubMed: 8440883]
Lipsky BA. Urinary tract infections in men: epidemiology, pathophysiology,diagnosis, and treatment. Ann Intern Med. 1989;110:138. [PubMed: 2462391]
Marshall JC, Christou NV, Horn R, Meakins JL. The microbiology of multiple organ failure: the proximalgastrointestinal tract as an occult reservoir of pathogens. Arch Surg. 1988;123:309. [PubMed: 3341911]
Mobley HLT, Chippendale GR. Hemagglutinin, urease, and hemolysin production byProteus mirabilis. J Infect Dis. 1990;161:525. [PubMed: 2179424]
Morris JG, Lin FYC, Morrison CB. et al. Molecular epidemiology of neonatal meningitis due toCitrobacter diversus: a study of isolates fromhospitals in Maryland. J Infect Dis. 1986;154:409. [PubMed: 3734491]
Nyström B. Impact of handwashing on mortality in intensive care: examinationof the evidence. Infect Control Hosp Epidemiol. 1994;15:435. [PubMed: 7963433]
Saito H, Elting L, Bodey GP, Berky P. Serratia bacteremia: review of 118 cases. Rev Infect Dis. 1989;11:912. [PubMed: 2602776]
Schaberg, DR, Culver, DH, Gaynes, RP Major trends in the microbial etiology of nosocomialinfection. Amer Med. 1991;91(suppl 3B):72S. [PubMed: 1928195]
Stamm, WE Catheter-associated urinary tract infections: epidemiology,pathogenesis, and prevention. Amer Med. 1991;91(suppl 3B):65S. [PubMed: 1928194]
Swartz MN. Hospital-acquired infections: diseases with increasingly limitedtherapies. Proc. Natl. Acad. Sci. USA. 1994;91:2420. [PMC free article: PMC43382] [PubMed: 8146133]
Toltzis P, Blumer JL. Antibiotic-resistant gram-negative bacteria in the critical caresetting. Pediatric Clin North Amer. 1995;42:687. [PubMed: 7761147]
van Saene HKF, Nunn AJ. et al. Viewpoint: survival benefit by selective decontamination of thedigestive tract (SDD). Infect Control Hosp Epidemiol. 1994;15:443. [PubMed: 7963435]
