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gram negative rods, entrobacteria


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gram negative rods, entrobacteria

  1. 1. BACTERIAL PATHOGENS: GRAM NEGATIVE RODS It comprises the following bacterial groups 1.Oxidase negative Enterobacteria a. Lactose-fermenters Escherichia spp. Klebsiella spp. b. Non-lactose fermenters Salmonella spp. Shigella spp.
  2. 2. Proteus spp. Yersinia 2. Oxidase Positive Pseudomonas Vibrio Campylobacter Helicobacter  ENTEROBACTERIA These are pathogens that belong to the large group of Gram negative rods. They are naturally found in the intestinal tract, in soil and water.
  3. 3. General Characteristics of Enterobacteria Often referred to as coliforms or enterobacilli Found as normal flora in intestinal tract of humans and animals. Enterobacteria are aerobes and facultative anaerobes, nonsporing and motile or non-motile. They are oxidase negative,reduce nitrate to nitrite, and ferment glucose with acid production(some also produce gas). Grow over a wide range of temperature in ordinary media. Release endotoxin from their cell wall. Some release exotoxin.
  4. 4. When lyzed, enterobacteria release endotoxin from their cell wall (feature of all Gram negative rods). Most of them have possessed three types of antigens which are used in serotyping, particularly salmonellae, shigellae, and E.coli. Cross-reactions however can occur, due to a sharing of antigens. O antigens: These are found in the bacterial cell wall. They are heat stable. K antigens: These are capsular polysaccharide antigens.
  5. 5. They surround the cell wall and can therefore interfere with the testing of underlying O antigens (K antigens can be heat-inactivated, enabling O antigens to be detected). The K antigens of some salmonellae, e.g. S. Typhi, are called Vi antigens. H antigens: These are flagellar protein antigens possessed by motile enterobacteria. They are heat labile (destroyed at 60–100 ºC). Salmonella serovars often produce two different antigenic types of H antigens, called phase 1 and phase 2.
  6. 6. 1.Genus Escherichia (a)Escherichia coli Pathogenicity E. coli causes: Urinary tract infections. E. coli is the commonest pathogen isolated from patients with cystitis. Recurring infections are common in women. Infections of wounds, peritonitis, sepsis and endotoxin induced shock. Meningitis and bacteraemia in neonates. E. coli capsular type K1 is associated with neonatal meningitis.
  7. 7. Diarrhoeal disease: infantile gastroenteritis, traveller’s diarrhoea, dysentery, and haemorrhagic diarrhoea which may progress to haemolytic uraemic syndrome. E. coli strains associated with diarrhoeal disease ETEC (Enterotoxigenic E. coli): Causes watery (secretory) diarrhoea due to the production of plasmid mediated toxins (LT, ST) in infants and adults, particularly in developing countries. It is often referred to as traveller’s diarrhoea. EPEC (Enteropathogenic E. coli): Causes vomiting,fever, and prolonged diarrhoea mainly in infants (less than 2 y). Due to bacteria adhering to epithelial cells, multiplying and causing lesions.
  8. 8. EIEC (Enteroinvasive E. coli): Causes dysentery (similar to shigellosis), fever and colitis, with blood, mucus, and many pus cells in faecal specimens. Due to bacteria invading and multiplying in epithelial cells. EHEC (Enterohaemorrhagic E. coli): Causes life- threatening haemorrhagic diarrhoea (colitis) in all ages, without pus cells, and often without fever. It can progress to haemolytic uraemic syndrome with renal failure. EaggEC (Enteroaggregative E. coli): Causes chronic watery diarrhoea and vomiting, mainly in children. Due to the bacteria adhering to tissue cells often in stacks(aggregates).
  9. 9. LABORATORY DIAGNOSIS Specimens: Depending on site of infection, specimens include urine, pus, faeces, cerebrospinal fluid (infants), and blood for culture. Morphology E. coli is a Gram negative usually motile rod. Inactive strains are nonmotile. A minority of strains are capsulate. Culture E. coli is an aerobe and facultative anaerobe. Optimum temperature for growth is 36–37 ºC with most strains growing over the range 18–44 ºC. Blood agar: E. coli produces 1–4 mm diameter colonies after overnight incubation.
  10. 10. The colonies may appear mucoid. Some strains are haemolytic. MacConkey agar and CLED agar: E. coli ferments lactose, producing smooth pink colonies on MacConkey agar and yellow colonies on CLED agar. Some strains (e.g. inactive strains) are late or non- lactose fermenting. XLD and DCA agar: Yellow colonies are produced on XLD agar. Growth of E. coli is usually inhibited on DCA agar. KIA (Kligler iron agar): Most strains of E. coli produce an acid deep and an acid slope with gas production and no H2S blackening (similar to other lactose fermenting coliforms).
  11. 11. Biochemical reactions Most strains of E. coli are; Indole positive Lysine decarboxylase (LDC) positive. Beta-glucuronidase positive Reduce nitrate to nitrite, giving a positive urine nitrite test. Citrate and H2S negative. Antimicrobial susceptibility Antimicrobials that are used to treat E. coli urinary and other infections include those with activity against Gram negative organisms such as sulphonamides, trimethoprim, cotrimoxazole,nalidixic acid, nitrofurantoin,
  12. 12. tetracycline, ampicillin,amoxycillin, cephalosporins, and aminoglycosides. In the treatment of E. coli diarrhoea, the use of antibiotics is in general only of minor importance. Rehydration of the patient is always the most important measure taken.