Gram Negative – Basic microbiology, parasitology, and immunology; nature, reproduction, growth, and transmission of common microorganisms and parasites in Bangladesh; prevention including universal precaution and immunization, control, sterilization, and disinfection; and specimen collections and examination. Students will have an understanding of common organisms and parasites caused human diseases and acquire knowledge about the prevention and control of those organisms.
Gram Negative
Escherichia coli (E. coli)
Escherichia coli (abbreviated as E. coli) are bacteria found in the environment, foods, and intestines of people and animals. E. coli are a large and diverse group of bacteria. Although most strains of E. coli are harmless, others can make you sick. Some kinds of E. coli can cause diarrhea, while others cause urinary tract infections, respiratory illness and pneumonia, and other illnesses.
Properties of E. coli:
- Gram-negative rod.
- Facultative anaerobe
- Ferments glucose.
- Most strains are lactose fermenters i.e. form pink colonies in Mac. Conkey’s agar media.
- Most strains are fimbriated / piliated.
- Motile with peritrichous flagella. Some strains may be non-motile.
- Catalase positive
- Oxidase negative.
Morphological and gram staining properties of Escherichia coli:
- Morphological property: They are bacilli with peritrichous flagella.
- Gram stain property: Gram negative.
Virulence Factors/Toxins and Enzymes/Determinants of Pathogenicity:
➤ Two Exotoxins (enterotoxins):
- Heat labile toxin (LT)
- Heat stable toxin (ST)
➤ Pili: They help the bacteria to adhere to the site of infection
➤ Capsule: Capsule interferes with phagocytosis.
➤ Endotoxin
➤ Verotoxin [Shiga- like toxin (SLT)]
Diseases Caused by E. coli:
E. coli causes diseases both within and outside the enteric tract.
➤ Intestinal tract infections: Diarrhoea, Dysentery
➤ Hemolytic-Uremic syndrome: This syndrome consists of hemolytic anemia, thrombocytopenia, and acute renal failure
➤ Urinary tract infections (UTI): Cystitis, Pyelonephritis (by certain O-serotype).
➤ Systemic infections:
- Wound infection
- Pneumonia, septicemia, abscess,
- Neonatal meningitis.
- Disseminated intravascular coagulation (DIC).
- Biliary tract infection.
Some species of E. coli (ETEC) cause watery, non-bloody diarrhoea and some species (EPEC, EHEC) cause bloody diarrhoea.
Pathogenesis/Mechanism of Diarrhoea by E. coli: (Enterotoxin mediated)
Mechanism of bloody diarrhoea (dysentery) by E. coli:
Adherence of E. coli (EHEC) to the cells of jejunum and ileum by pili.
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Invasion of epithelium of large intestine
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Bloody diarrhoea,
Classification of E. coli on the Basis of Diarrohea Production:
➤ Enterotoxigenic E. coli (ETEC): Traveler’s diarrhoea and infantile enteritis.
➤ Enteropathogenic E. coli (EPEC): Watery diarrhoea of long duration, mostly in infants, often in developing countries.
➤ Entero-hemorrhagic E. coli (EHEC) / Verotoxin producing E. coli (VTEC):
- Bloody diarrhoea.
- Hemorrhagic colitis.
- Hemolytic uremic syndrome.
➤ Enteroinvasive E. Coli (EIEC): Bloody diarrhoea.
➤ Enteroadherent E. coli (EAEC)/Enteroaggregative E. coli (EAggEC): Persistent watery diarrhoea in children and patients infected with HIV.
Lab. Diagnosis of Watery Diarrhoea by E. coli:
Principle:
The diagnosis is based on demonstration of the causative organism by microscopic examination and isolation and identification by culture. Immunological test and some other tests are also helpful.
Steps:
➤ Specimen collection:
- Freshly passed stool
- Rectal swab
➤ Microscopic examination:
- Gram staining: Finding- Gram-negative rods are found.
➤ Isolation and identification from culture:
- Mac Conkey’s agar.
- Incubated at 37°C.
- Finding: Form pink colonies (due to fermentation of lactose).
➤ Immunological test: Sometimes helpful.
➤ Special tests: For ETEC, EPEC, VTEC done in reference Lab.
Salmonella
Morphology of Salmonella:
- Gram-negative and non-acid-fast rods.
- Facultative anaerobes.
- Ferment glucose.
- Do not ferment lactose.
- Catalase positive.
- Oxidase negative.
- They form colour-less colonies on Mac Conkey’s agar media.
- They produce H₂S.
- Most strains are motile with peritrichous flagella
- Fimbriae present.
- Non-spore forming.
Clinically Important Species:
- Salmonella typhi
- Salmonella paratyphi A
- Salmonella paratyphi B
- Salmonella paratyphi C
- Salmonella typhimurium
- Salmonella choleraesuis
- Salmonella enteritidis
- Salmonella newport
- Salmonella dublin
Classification of Salmonella According to Pathogenicity:
➤ Salmonella that cause enteric fever:
- Salmonella typhi
- Salmonella paratyphi – A, B, C
➤ Salmonella that cause food poisoning (enterocolitis):
- Salmonella typhimurium
- Salmonella enteritidis
➤ Salmonella that cause septicaemia:
- Salmonella choleraesuis
Diseases Caused by Salmonella Species:
- Enteric fevers (Typhoid fever and paratyphoid fever).
- Food poisoning / Gastroenteritis (by S. typhimurium, S. enteritidis, S. newport, S. dublin).
- Enterocolitis in children.
- Septicemia (by Salmonella choleraesuis)
- Asymptomatic carrier state
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