Enterobacteriaceae

Enterobacteriaceae :

Enterobacteriaceae is a large family of bacteria that includes many genera, such as Escherichia, Salmonella, and Klebsiella. These bacteria are commonly found in the intestines of humans and animals, and they play significant roles in both health and disease. Some members of this family are important pathogens, causing infections like urinary tract infections, gastroenteritis, and septicemia, while others are part of the normal microbiota. Enterobacteriaceae are typically Gram-negative and can be identified through biochemical tests, including lactose fermentation and hydrogen sulfide production.

Enterobacteriaceae:

Definition: Enterobacteriaceae is a family of Gram-negative bacteria that includes a wide variety of genera and species, many of which are significant in human health and disease.

General Characteristics:

Gram Stain: Gram-negative.

Shape: Rod-shaped (bacilli).

Oxygen Requirement: Facultative anaerobes.

Fermentation: Capable of fermenting glucose.

Motility: Some members are motile with flagella, while others are non-motile.

Biochemical Features:

1.Catalase-positive.

2.Oxidase-negative (most).

3.Can produce hydrogen sulfide (H2S).

Common Genera

Escherichia:

E. coli: Normal gut flora; some strains can cause foodborne illness.

Salmonella:

Salmonella enterica: Causes salmonellosis, often through contaminated food or water.

Klebsiella:

Klebsiella pneumoniae: Associated with pneumonia, UTIs, and other infections, especially in immunocompromised individuals.

Enterobacter:

Enterobacter cloacae: Can cause hospital-acquired infections.

Serratia:

Serratia marcescens: Known for nosocomial infections.

Proteus:

Proteus mirabilis: Often associated with urinary tract infections

Shigella:

Causes shigellosis, characterized by diarrhea and abdominal pain.

Pathogenicity

Virulence Factors:

Endotoxins: Lipopolysaccharides (LPS) in the outer membrane can trigger strong immune responses.

Exotoxins: Some species produce toxins that can damage host cells.

Adhesins: Help bacteria adhere to host tissues.

Invasion Factors: Facilitate entry into host cells.

Diseases:

Gastroenteritis, urinary tract infections, sepsis, and respiratory infections.

Diagnosis

Culturing: Samples from blood, urine, stool, etc.

Biochemical Tests: Lactose fermentation, gas production, H2S production.

Molecular Methods: PCR and other nucleic acid-based techniques for identification.

Treatment:

Antibiotics: Choice depends on susceptibility; some strains have developed resistance (e.g., ESBL-producing organisms).

Supportive Care: Hydration and electrolyte management for gastrointestinal infections.

 Prevention :

Hygiene Practices: Proper handwashing, cooking food thoroughly, and safe food handling.

Vaccines: Currently, no broadly effective vaccines exist for most Enterobacteriaceae, but research is ongoing.

 Conclusion:

Enterobacteriaceae encompasses a diverse group of bacteria with significant implications for public health. Understanding their characteristics, pathogenic mechanisms, and methods for prevention and treatment is crucial for managing infections effectively.

General Characteristics: Enterobacteriaceae are among the largest bacteria, with parallel sides and rounded ends. Forms range from large coccobacilli to elongated, filamentous rods. The organisms do not form spores or demonstrate acid-fastness. The cell wall, cell membrane, and internal structures are morphologically similar for all Enterobacteriaceae. The outer membrane lipopolysaccharide (LPS) is called the O antigen. Its antigenic specificity is determined by the composition of the sugars that form the long terminal polysaccharide side chains linked to the core polysaccharide and lipid A. Cell surface polysaccharides may form a well-defined capsule or an amorphous slime layer and are termed the K antigen (from the Danish Kapsel, capsule). Motile strains have protein peritrichous flagella, which extend well beyond the cell wall and are called the H antigen. Structure of Gram negative bacteria Many Enterobacteriaceae have surface pili (fimbriae).Enterobacteriaceae grow readily on simple media, often with only a single carbon energy source. All Enterobacteriaceae ferment glucose, reduce nitrates to nitrites, and are oxidase negative.

                                E.coli 

Escherichia Coli Most strains of E coli ferment lactose rapidly and produce indole. Aerobic and facultative anaerobe and grows on ordinary culture medium at 37ºC (10-40ºC) in 1824 hrs. Most E coli express Pili which play a role in virulence as mediators of attachment to human epithelial surfaces. Toxins The α-hemolysin is a pore-forming cytotoxin that inserts into the plasma membrane of a wide range of host cells in a manner similar to streptolysin O and Staphylococcus aureus α-toxin. The toxin causes leakage of cytoplasmic contents and eventually cell death. The more recently discovered cytotoxic necrotizing factor (CNF) is often produced in concert with α-hemolysin. Shiga-type toxin – also called the verotoxin -produced by enterohemorrhagic strains of E. coli (EHEC) – is cytotoxic, enterotoxic, neurotoxic, and may cause diarrhea and ulceration of the G.I. tract. There are two types shiga-like toxin 1 and shiga-like toxin 2. Labile toxin (LT) is also an A-B toxin. The B subunit binds to the cell membrane, and the A subunit catalyzes the ADP-ribosylation of a regulatory G protein located in the membrane of the intestinal epithelial cell. If the cell is an enterocyte, the result is the stimulation of chloride secretion out of the cell and the blockage of NaCl absorption. The net effect is the secretion of water and electrolytes into the bowel lumen. Stable toxin (ST) toxin is a small peptide that binds to a glycoprotein receptor, resulting in the activation of a membrane-bound guanylate cyclase. The subsequent increase in cyclic GMP concentration causes an LT-like net secretion of fluid and electrolytes into the bowel lumen. Colicins (bacteriocins):They are bactericidal substances are produced by certain bacteria active against some other strains of the same or closely related species. Bacteriocin producing strains are resistant to their own bacteriocins. These substances are proteins in chemical nature & are antibiotics –like substances.

 Proteus and Morganella :

Proteus , Morganella, Providencia group Proteus Member of the family Enterobacteriaceae .Genus Proteus produces infections in humans only when the bacteria leave the intestinal tract. Species of Proteus are found widely distributed in soil, on vegetables, in sewage and faeces . Morphology : They are Gram negative bacilli ,motile with peritrichus flagella . The most important pathogens in this genus are : P .vulgaris , Pathogenesis & P. mirabilis . After E.coli P. mirabilis it is the commonest cause of urinary tract infections especially of young males & the elderly of both sexes. Proteus species has also been recovered from infected wounds , abscesses , from cases of otitis media ,meningitis ,septicaemia , osteomyelitis and focal lesions of diabetic patients . Proteus species produce urease ,resulting in rapid hydrolysis of urea with liberation of ammonia and CO2.Thus in urinary tract infections with proteus the urine becomes alkaline ,promoting stone formation .The rapid motility of this bacteria may contribute to its invasion of the urinary tract . P.vulgaris is an important nosocomial pathogens. Diagnostic Laboratory Specimens: urine, wound ,blood ,pus and sputum.