Small Gram-negative non-sporeforming enteric bacilli
All Enterobacteriaciae:
ferment glucose with acid production
reduce
nitrates (NO3 to NO2 or all the way to N2)
are oxidase negative
All are aerobic but can be facultatively anaerobic
Motile via peritrichous flagella except Shigella and Klebsiella which are non-motile
Capsule, slime layer, or neither
Possess fimbriae (pili)
Complex cell wall
Antigenic Structure: plays an important role for some species in epidemiology and classification
K (capsular) antigens: capsular polysaccharide, particularly heavy in Klebsiella
Over 30 genera and 120 species
More than 95% of clinically significant strains fall into 10 genera and less than 25 species
Some members of the Enterobacteriaceae are true pathogens
Most members of the Enterobacteriaceae are opportunistic or cause secondary infections of wounds, the urinary and respiratory tracts, and the circulatory system
Gram-negative sepsis
- Life-threatening
- Usually nosocomial
- Commonly caused by E. coli
- Clinical Progression:
- Early Phase (REVERSIBLE)
Second Phase (REVERSIBLE)
Decrease in arterial resistance; Increased cardiac output
Kinins (protein) in plasma due to tissue damage, endotoxin, or antigen-antibody complexesThird Phase (IRREVERSIBLE) Vascular collapse with organ failure
Increase in arterial resistance; Decreased cardiac output
- Endotoxin induces DIC which leads to hemorrhage and death
Urinary tract infections
- Greatest incidence in young individuals and middle-aged females
- Incidence increases with age in males
- Most commonly caused by E. coli
- Diagnosis by microscopic and cultural examination of urine
- Obtain urine by catheter through urethra into bladder, suprapubic tap
- Nosocomial; Spread by personnel and equipment
- Frequently caused by K. pneumoniae
- Often in middle-aged males who abuse alcohol
- Difficult to diagnose because of commensals in sputum
Abdominal sepsis
- Caused by flora of the gastrointestinal tract
- Infections usually polymicrobi
Meningitis
- Usually nosocomial
- Frequently caused by E. coli
- Diagnosis by microscopic and cultural examination of cerebrospinal fluid
Spontaneous bacterial peritonitis
- Usually in patients with liver ailments
- Commonly caused by E. coli, but also anaerobes and Gram-positive cocci (S. pneumoniae)
Endocarditis
- Vascular endocardial surface inflammation
- Mostly caused by Gram-positive cocci, but 1-3% caused by aerobic Gram-negative rods
- Diagnosis by blood culture
- Difficult to treat; treatment is of long duration
Found in soil, water and decaying matter
Some of the normal inhabitants of the small and large intestine included in the family: therefore, are sometimes referred to as enteric bacilli or simply enterics
Enterics are responsible for a majority of nosocomial infections
About two million patients per year in the United States are estimated to acquire nosocomial infections
Approximately 5% to 10% of the total hospital population acquire such infections
Endotoxin: important virulence factor with wide-ranging effects on host
Capsule: antiphagocytic
Antigenic phase variation: capability to alternately express or not express either capsule or flagella and thus avoid host immunity
Sequestration of nutritional factors, in particular, iron by production of siderophores which are extracellular iron-chelating compounds (e.g., enterobactin, aerobactin)
Iron is important compound for both host and pathogen and is limited in supply and thus must be competed for
Much of iron in host body is sequestered in heme proteins (e.g., hemoglobin, myoglobin) and in iron-chelating proteins (e.g., transferrin, lactoferrin)
Resistance to serum killing: many bacteria are inherently sensitive to nonspecific bloodborne components and to circulating complement and the resultant complement-mediated clearance, but Enterobacteriaceae and other bloodborne pathogens can resist such killing
True pathogenic members of the Enterobacteriaceae may possess specific virulence features, which are unique to individual genera or species
Adhesins
Exotoxins, (e.g., enterotoxins which act in the small intestine)
Antigens which stimulate antibody production by the host include:
Specimens whether pus, tissue, sputum, fluids, rectal swabs, or feces should be cultured immediately or placed on special media to prevent overgrowth
Special isolation media: contain various substances including indicators, inhibitors, etc.
Media and tests to differentiate the genera of the family
Tests that divide species of the genera, e.g., patterns of acid production from various carbohydrates
Various species differ in the carbohydrates from which acid may be produced and end products that may be formed from various substrates
Culture:
Colony morphology: moist, gray (except Serratia marcescans which appears red) smooth colonies on non-selective media
Special differential and selective media used for separation of genera and species
Some strains are beta hemolytic on blood agar
Major problem
Various options; must consider resistance
Citrobacter: environmental sources and in feces of man and animals
Can infect any body site opportunistically, but most isolates from the urinary tract; neonatal meningitis and brain abscesses by Citrobacter diversus
Citrobacter freundii can be enterotoxigenic
Antigenic structure: O, H, K antigens
Enterobacter: inhabit soil and water and to a lesser extent, the human intestine
Similar to and must be distinguished from Klebsiella in specimens
Cultural characteristics: motile, four species biochemically
Antigenic structure: O, H, K antigens
Disease in body tissues, most frequently urinary tract; Enterobacter cloacae found most often
Strains of E. cloacae are enteroxigenic and may be antibiotic resitant
Treat with any antimicrobials used for enterobacterial diseases
Serratia: inhabitants of water and soil
Major agent in nosocomial infections; three species represent major human pathogens
Cultural characteristics: produces DNase, red pigment production (in nature, but rarely in strains isolated from infection) enhanced at 25° C
Antigenic structure: O, H, K antigens
Morganella / Providencia: airborne organisms that frequently colonize burn wounds and are often antibiotic resistant in vivo
Antigenic structure: O, H, K antigens
Morganella morganii: formerly Proteus morganii, causes infections similar to Proteus
Edwardsiella: rarely causes wound infections; also rarely seen in sepsis, meningitis, and gastroenteritis in humans
Produces hydrogen sulfide, but
doesn't ferment lactose
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