Citrobacter spp. (C. freundii, C. koseri, C. braakii)

Citrobacter organisms are inhabitants of the intestinal tract. The most common clinical manifestation in patients as a result of infection occurs in the urinary tract. However, additional infections, including septicemias, meningitis, brain abscesses, and neurologic complications, have been associated with Citrobacter spp. Transmission is typically person to person. Table 1 provides an outline of the biochemical differentiation of the most common clinically isolated Citrobacter species. C. freundii may harbor inducible AmpC genes that encode resistance to ampicillin and first-generation cephalosporins.

Table1.  Biochemical Differentiation of Citrobacter Species

 Cronobacter sakazakii

Cronobacter sakazakii, formerly Enterobacter sakazakii, is a pathogen associated with bacteremia, meningitis, and necrotizing colitis in neonates. The organism produces a yellow pigment that is enhanced by incubation at 25°C. C. sakazakii may be differentiated from Enterobacter spp. as Voges-Proskauer, arginine dihydrolase, ornithine decarboxylase positive. In addition, the organism displays the following fermentation reactions: D-sorbitol negative, raffinose positive, L-rhamnose positive, melibiose positive, D-arabitol negative, and sucrose positive. C. sakazakii is intrinsically resistant to ampicillin and first- and second generation cephalosporins as a result of an inducible AmpC chromosomal β-lactamase. Mutations to the AmpC gene may result in overproduction of β-lactamase, conferring resistance to third-generation cephalosporins.

 Edwardsiella tarda

Edwardsiella tarda is infrequently encountered in the clinical laboratory as a cause of gastroenteritis. The organism is typically associated with water harboring fish or turtles. Immunocompromised individuals are particularly susceptible and may develop serious wound infections and myonecrosis. Systemic infections occur in patients with underlying liver disease or conditions resulting in iron overload.

Enterobacter spp. (E. aerogenes, E. cloacae, E. gergoviae, E. amnigenus, E. taylorae)

Enterobacter spp. are motile lactose fermenters that produce mucoid colonies. Enterobacter spp. are reported as one of the genera listed in the top 10 most frequently isolated health care–associated infections by the National Healthcare Safety Network. The infections are typically associated with contaminated medical devices, such as respirators and other medical instrumentation. The organism has a capsule that provides resistance to phagocytosis. Enterobacter spp. may harbor plasmids that encode multiple antibiotic resistance genes, requiring antibiotic susceptibility testing to identify appropriate therapeutic options.

 Escherichia coli (UPEC, MNEC, ETEC, EIEC, EAEC, EPEC and EHEC)

Molecular analysis of E. coli has resulted in the classification of several pathotypes as well as commensal strains. The genus consists of facultative anaerobic, glucose fermenting, gram-negative, oxidase-negative rods capable of growth on MacConkey agar. The genus contains motile (peritrichous flagella) and nonmotile bacteria. Most E. coli strains are lactose fermenting, but this function may be delayed or absent in other Escherichia spp.

Isolates of extraintestinal E. coli strains have been grouped into two categories: uropathogenic E. coli (UPEC) and meningitis/sepsis–associated E. coli (MNEC). UPEC strains are the major cause of E. coli–associated urinary tract infections. These strains contain a variety of pathogenicity islands that code for specific adhesions and toxins capable of causing disease, including cystitis and acute pyelonephritis. MNEC causes neonatal meningitis that results in high morbidity and mortality. Eighty percent of MNEC strains test positive for the K1 antigen. The organisms are spread to the meninges from a blood infection and gain access to the central nervous system via membrane-bound vacuoles in microvascular endothelial cells.

As mentioned, intestinal E. coli may be classified as enterohemorrhagic (or serotoxigenic [STEC], or verotoxigenic [VTEC]), enterotoxigenic, enteropatho genic, enteroinvasive, or enteroaggregative. EHEC is recognized as the cause of hemorrhagic diarrhea, colitis, and hemolytic uremic syndrome (HUS). HUS, which is characterized by a hemolytic anemia and low platelet count, often results in kidney failure and death. Unlike in dysentery, no white blood cells are found in the stool. Although more than 150 non-O157 serotypes have been associated with diarrhea or HUS, the two most common are O157:H7 and O157:NM (nonmotile). The O antigen is a component of the lipopolysaccharide of the outer membrane, and the H antigen is the specific flagellin associated with the organism. ETEC produces a heat labile enterotoxin (LT) and a heat-stable enterotoxin (ST) capable of causing mild watery diarrhea. ETEC is uncommon in the United States but is an important pathogen in young children in developing countries. EIEC may produce a watery to bloody diarrhea as a result of direct invasion of the epithelial cells of the colon. Cases are rare in the United States. EPEC typically does not produce exotoxins. The pathogenesis of these strains is associated with attachment and effacement of the intestinal cell wall through specialized adherence factors. Symptoms of infection include prolonged, nonbloody diarrhea; vomiting; and fever, typically in infants or children. EAEC has been isolated from a variety of clinical cases of diarrhea. The classification as aggregative results from the control of virulence genes associated with a global aggregative regulator gene, AggR, responsible for cellular adherence. EAEC-associated stool specimens typically are not bloody and do not contain white blood cells. Inflammation is accompanied by fever and abdominal pain.

Ewingella americana

Ewingella americana has been identified from blood and wound isolates. The organism is biochemically inactive, and currently no recommended identification scheme has been identified.

 Hafnia alvei

Hafnia alvei (formerly Enterobacter hafniae) has been associated with gastrointestinal infections. The organism, resides in the gastrointestinal tract of humans and many animals It is a motile non–lactose fermenter and is often isolated with other pathogens. Most infections with H. alvei are indentified in patients with severe underlying disease (e.g., malignancies) or after surgery or trauma. However, a distinct correlation with clinical signs and symptoms has not been clearly developed, probably because of the lack of identified clinical cases. Treatment is based on antimicrobial susceptibility testing.

Klebsiella spp. (K. pneumoniae, K. oxytoca)

Klebsiella spp. are inhabitants of the nasopharynx and gastrointestinal tract. Isolates have been identified in association with a variety of infections, including liver abscesses, pneumonia, septicemia, and urinary tract infections. Some strains of K. oxytoca carry a heat labile cytotoxin, which has been isolated from patients who have developed a self-limiting antibiotic-associated hemorrhagic colitis. K1 capsular–containing K. pneumoniae organisms are increasingly isolated from community-acquired pyogenic liver abscess worldwide. All strains of K. pneumoniae are resistant to ampicillin. In addition, they may demonstrate multiple antibiotic resistance patterns from the acquisition of multidrug-resistant plasmids, with enzymes such as carbapenemase.

 Morganella spp. (M. morganii, M. psychrotolerans)

Morganella spp. are found ubiquitously throughout the environment and are often associated with stool specimens collected from patients with symptoms of diarrhea. They are normal inhabitants of the gastrointestinal tract. M. morganii is commonly isolated in the clinical laboratory; however, its clinical significance has not been clearly defined. Morganella spp. are deaminase positive and urease positive.

 Pantoea agglomerans

Pantoea agglomerans appears as a yellow-pigmented colony and is lysine, arginine, and ornithine negative. In addition, the organism is indole positive and mannitol, raffinose, salicin, sucrose, maltose, and xylose negative. The organism is difficult to identify using commercial or traditional biochemical methods due to the high variability of expression in the key reactions. Sporadic infections can occur due to trauma from objects contaminated with soil or from contaminated fluids (i.e., IV fluids).

Plesiomonas shigelloides

Plesiomonas shigelloides is a fresh water inhabitant that is transmitted to humans by ingestion of contaminated water or by exposure of disrupted skin and mucosal surfaces. P. shigelloides can cause gastroenteritis, most frequently in children, but its role in intestinal infections is still unclear.

 P. shigelloides is unusual in that it is among the few species of clinically relevant bacteria that decarboxylate lysine, ornithine, and arginine. It is important to distinguish Aeromonas spp. from P. shigelloides., since both are oxidase positive. This is accomplished by using the string test . The DNase test may also be used to differentiate these organisms. Aeromonas spp. are DNase positive and Plesiomonas organisms are DNase negative.

 Proteus spp. (P. mirabilis, P. vulgaris, P. penneri) and Providencia spp. (P. alcalifaciens, P. heimbachae, P. rettgeri, P. stuartii, P. rustigianii)

The genera Proteus and Providencia are normal inhabitants of the gastrointestinal tract. They are motile, non-lactose fermenters capable of deaminating phenylalanine. Proteus spp. are easily identified by their classic “swarming” appearance on culture media. However, some strains lack the swarming phenotype. Proteus has a distinct odor that is often referred to as a “chocolate cake” or “burnt chocolate” smell. For safety reasons, smelling plates is strongly discouraged in the clinical laboratory. Because of its motility, the organism is often associated with urinary tract infections; however, it also has been isolated from wounds and ears. The organism has also been associated with diarrhea and sepsis.

Providencia spp. are most commonly associated with urinary tract infections and the feces of children with diarrhea. These organisms may be associated with nosocomial outbreaks. No clear clinical association exists when these organisms are isolated.

Serratia spp. (S. marcescens, S. liquefaciens group)

Serratia spp. are known for colonization and the cause of pathagenic infections in health care settings. Serratia spp. are motile, slow lactose fermenters, DNAse, and orthonitrophenyl galactoside (ONPG) positive. Serratia spp. are ranked the twelfth most commonly isolated organism from pediatric patients in North America, Latin America, and Europe. Transmission may be person to person but is often associated with medical devices such as urinary catheters, respirators intravenous fluids, and other medical solutions. Serratia spp. have also been isolated from the respiratory tract and wounds. The organism is capable of survival under very harsh environmental conditions and is resistant to many disinfectants. The red pigment (prodogiosin) produced by S. marcescens typically is the key to identification among laboratorians, although pigment-producing strains tend to be of lower virulence. Other species have also been isolated from human infections. Serratia spp. are resistant to ampicillin and first-generation cephalosporins because of the presence of an inducible, chromosomal AmpC β-lactamase. In addition, many strains have plasmid-encoded antimicrobial resistance to other cephalosporins, penicillins, carbapenems, and aminoglycosides.

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تناول الجبن.. مفيد للجسم أم خطر يهدد صحتك؟

طبيبة أسنان تحذر.. لا تتجاهل هذه العلامة بعد التنظيف

يجب أن تأكل المزيد من الشمام هذا الصيف.. إليك السبب ؟

تأثير صحي كبير.. كم بيضة يجب أن تتناول يوميا؟

المزيد

الآخبار التكنلوجية

أداء خلايا البيروفسكايت الشمسية تحت الماء قد يزيد كفاءة تحويل الكهرباء

مسافة الأمان: أساس القيادة الآمنة وتجنب الحوادث

هل القيادة السريعة تعني استهلاك اكثر للوقود؟

لماذا يعتبر كوكب المريخ غير صالح للحياة؟

المزيد

مواضيع ذات صلة

Cultivation of Enterobacteriaceae

Primary Intestinal Pathogens of Enterobacteriaceae

Vibrio

Salmonella

Escherichia coli

Laboratory Diagnosis for Nocardia, Streptomyces, Rhodococcus, and Similar Organisms

Epidemiology and Pathogenesis of Nocardia, Streptomyces, Rhodococcus, and Similar Organisms

General Characteristics of Nocardia, Streptomyces, Rhodococcus, and Similar Organisms

Pathogenesis of Streptococcus pneumoniae

Treatment and Prevention of Staphylococcus aureus

Pathogenesis of Staphylococcus aureus

Direct Detection Methods for Erysipelothrix, Lactobacillus, and Similar Organisms