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Escherichia coli and Klebsiella spp belong to a group of bacteria called Enterobacteriaceae. The Enterobacteriaceae are a large, heterogeneous group of gram-negative bacilli whose natural habitat is the intestinal tract of humans and animals. They are gram negative, non-spore forming bacilli that grow both aerobically and anaerobically on ordinary laboratory media, including MacConkey’s lactose-bile salt agar, and conform to the following definition:

Typical and distinguishing characteristics of this family are that they (1) are non-spore-forming aerobes capable of anaerobic growth (facultative anaerobes), (2)

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reduce nitrates to nitrites (with some exceptions), (3) do not liquefy alginate, (4) ferment glucose to acid with or without gas, (5) are oxidase-negative, (6) do not have growth enhancement by NaCl, and (7) can be either motile (with peritrichous flagella) or nonmotile.¹⁰¹

The family includes many genera such as Escherichia, Shigella, Salmonella, Enterobacter, Klebsiella, Serratia, Proteus, and others. According to the current edition of Bergey’s manual, there are over 31 genera and 139 species of gram negative bacilli belonging to the family Enterobacteriaceae.¹⁰² These enteric organisms are associated with the enteric tract but could be pathogenic. Gram negative bacilli belonging to the family Enterobacteriaceae produce a variety of toxins and other virulence factors and are the most frequently encountered bacterial isolates recovered from clinical specimen.

Escherichia coli

Escherichia coli is the most frequent cause of some of the most common bacterial infections, including urinary tract infections, bacteremia, and bacteria-related traveler's diarrhoea. It is also a leading cause of neonatal meningitis and can cause a variety of other clinical infections, including pneumonia. Strains of E. coli normally colonize the large intestine of humans. In a study of the various serogroups and clones recovered from the faeces of healthy adults, it was found that strains typically associated with enteropathogenic or enterohemorrhagic

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disease were not recovered. Also faecal strains in general had fewer virulence-associated factors than what were found in isolates virulence-associated with infections of the urinary tract, meninges, and blood stream.¹⁰¹ Although the most common cause of urinary tract infection, capable of causing a wide range of illness, ranging from uncomplicated urethritis to symptomatic cystitis or pyelonephritis to sepsis,¹⁰³ only a few serogroups (uropathogenic E. coli clones) of the organism, 01, 02, 04, 06, 07, 075, 0150, cause a high proportion of infections.¹⁰⁴ E. coli can remain in the urinary tract by adhering to the epithelial receptor cells lining the urinary tract and are able to avoid elimination by the flushing action of voided urine. A variety of virulence factors contributing to uropathogenesis of E. coli have been recognized, including specific fimbriae, hemolysin production, presence of colicin V plasmids, certain capsular and lipopolysaccharide antigens.

The typical uropathogenic strain of E. coli possesses factors believed to be necessary for the pathogenesis of urinary tract infection. These include:- increased adherence to vaginal and uroepithelial cells, fimbriae or pilli; resistance to serum bactericidal activity; presence of aerobactin; cytotoxic necrotizing factor type 1; a higher quantity of K antigen; hemolysin production.^102,105

Among clinical isolates these bacteria can be motile or nonmotile, and most ferment lactose. Other biochemical tests frequently used to identify E. coli in the clinical microbiology laboratory are as follows: the methyl red reaction is positive,

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the Voges-Proskauer (VP) test is negative, urease and phenylalanine deaminase activity is absent, H2S is not produced, citrate cannot be used as the sole carbon source, and the organism will not grow in the presence of potassium cyanide (KCN).¹⁰²

Klebsiella species

The genus Klebsiella constitutes a group of nonmotile members of the enterobacteriaceae that have been traditionally speciated into K. pneumoniae, K.

ozaenae and K. rhinoscleromatis. A notable characteristic of Klebsiella spp. is their large appearance by Gram stain. They are usually capsulate and can be recognized by their large, grayish–white, mucoid colonies on laboratory medium.

The traditional grouping is based on the biochemical reactions of bacteria in the genus. Biochemical reactions of Klebsiella are as follows: They are phenylalanine deaminase negative, do not produce H2S on Tripple sugar iron agar and do not liquefy gelatin. Contrasting with Enterobacter spp, they are ornithine decarboxylase negative. They are indole-negative, and capable of growing in KCN and using citrate as sole carbon source, and they give a positive VP reaction. They ferment a wide range of carbohydrates.¹⁰²

The taxonomy of Klebsiella is characterized by a nomenclature reflecting its colourful taxonomic history. Originally, the medical importance of the genus Klebsiella (family Enterobacteriaceae)led to its being subdivided into three species

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corresponding to the diseases they caused: K. pneumoniae, K. ozaenae, and K. rhinoscleromatis. As the taxonomy became increasingly refined due to the development of new methods such as numerical taxonomy, the species classification in this genus was continually revised. In time, three main classificationsemerged, those of Cowan, Bascomb, and Orskov. According to the Orskov classification, all these three bacteria belong to one species (K pneumoniae) and are therefore classified as K. pneumoniae subsp. pneumoniae, K. pneumoniae subsp. Ozaenae and K. pneumoniae subsp. rhinoscleromatis.

Additional species are K. oxytoca, K. terrigena, K. planticola (syn. K. trevisanii), K. ornithinolytica.¹⁰⁶

K. pneumoniae subsp. pneumoniae (simply called K. pneumoniae) is the most frequently isolated from clinical specimens. Factors that may be contributory to its virulence include:

Production of fimbriae. Most strains of K. pneumoniae produce two different types of fimbriae that mediate adhesion to host cells. Type 1 and Type 3. Type 1 has been associated with urinary tract infections and type 3 may be associated with catheter-associated bacteriuria.¹⁰⁷

Extensive production of capsular polysaccharides (K antigen) preventing phagocytosis and helping to retard leukocyte migration into infected areas.¹⁰¹

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Regarded as the main determinant of klebsiella pathogenicity; ¹⁰¹ different capsular types have been reported.¹⁰⁸

Resistance to the bactericidal effect of human serum. In K. pneumoniae, serum resistance properties are more common among isolates from clinical specimens than in faecal or environmental isolates.¹⁰⁹

Production of high affinity iron-chelating siderophores. The contribution of the hydroxamate-type siderophore, aerobactin, to virulence has been demonstrated clearly.¹¹⁰

Klebsiella spp. are ubiquitous in nature. They probably have two common habitats, one being the environment, where they are found in surface water, sewage, and soil and on plants, and the other being the mucosal surfaces of mammals such as humans, horses, or swine, which they colonize. In humans, K. pneumoniae is present as a saprophyte in the nasopharynx and in the intestinal tract. Carrier rates differ considerably from study to study. The detection rate in stool samples ranges from 5 to 38%, while rates in the nasopharynx range from 1 to 6%. These carrier rates change drastically in the hospital environment, where colonization rates increase in direct proportion to the length of stay. Even hospital personnel have elevated rates of Klebsiella carriage. Reported carrier rates in hospitalized patients are 77% in the stool, 19% in the pharynx, and 42% on the hands of patients.¹¹¹ Carriers are prone to infections when host defenses are lowered.

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Infections caused by K. pneumoniae include lobar pneumonia (especially in immunocompromised and other high risk patient groups) ¹¹¹ and UTIs.¹¹²

The vast majority of Klebsiella infections, however, are associated with hospitalization. Klebsiella has been incriminated in 8% of all nosocomial bacterial infections. The most common foci for such infections are the urinary tract, lower respiratory tract, biliary tract, and surgical wound sites, in that order. Invasive devices found in hospitalized patients, particularly urinary catheters, endotracheal tubes, and intravenous catheters, markedly increase the disposition to nosocomial infections, particularly gram-negative rods. Like most gram-negative organisms found in the hospital environment, Klebsiella is characteristically resistant to multiple antibiotics. Klebsiella is naturally resistant to ampicillin and carbenicillin, and increasing acquisition of R plasmids is mediating drug resistance to cephalosporins and aminoglycosides with increased frequency.¹⁰¹ Especially feared are epidemic hospital infections caused by multidrug-resistant strains. Strains that produce ESBLs, which make them resistant to extended-spectrum cephalosporins, have evolved. The hallmark of these strains, resistance to ceftazidime, is observed in both K. pneumoniae and K. oxytoca isolates. ¹¹¹