L9 Enterobacteriaceae_student (1)

Enterobacteriaceae are a family of Gram-negative bacteria that are typically rod-shaped. They are facultative anaerobes, meaning they can grow in both aerobic and anaerobic environments. They ferment glucose and are often found in the intestinal tract of humans and animals. Many members are motile due to the presence of flagella, and they can produce lactose and non-lactose fermenters. Additionally, they are known for their ability to produce urease and indole in some species.

Enterobacteriaceae are primarily found in the gastrointestinal tract of humans and animals, serving as normal flora. They can also be found in soil, water, and vegetation. Epidemiologically, they are significant as they can cause infections in humans, particularly in hospital settings. The spread of these bacteria can occur through contaminated food and water, as well as through person-to-person contact. Certain species, like E. coli, are associated with outbreaks of gastroenteritis and other infections.

Enterobacteriaceae are medically significant due to their role in various infections such as urinary tract infections, gastroenteritis, and sepsis. Some species, like Klebsiella pneumoniae and Escherichia coli, are known for their antibiotic resistance, making infections difficult to treat. They are also involved in nosocomial infections, particularly in immunocompromised patients. Understanding their medical significance is crucial for effective treatment and infection control.

Enterobacteriaceae exhibit several key biochemical properties:

1. Fermentation of glucose: They can ferment glucose to produce acid and gas.
2. Lactose fermentation: Some members, like E. coli, can ferment lactose, while others, like Salmonella, cannot.
3. Urease production: Some species produce urease, which hydrolyzes urea to ammonia.
4. Indole production: Certain species can produce indole from tryptophan.
5. Catalase positive: They produce the enzyme catalase, which breaks down hydrogen peroxide.
6. Oxidase negative: They do not produce the enzyme oxidase, which differentiates them from other Gram-negative bacteria.

Gram-negative rods can be classified based on their oxygen requirements into the following categories:

1. Aerobic

   • Examples: Pseudomonas, Neisseria

2. Microaerophilic

   • Examples: Campylobacter

3. Facultative Anaerobic

   • Examples: Enterobacteriaceae (e.g., E. coli, Klebsiella, Salmonella, Shigella)

4. Anaerobic

   • Examples: Bacteroides, Veillonella

Overall, there are over 25 genera of gram-negative rods.

Obligate aerobes require oxygen for growth and show concentrated growth at the top of the nutrient broth.

Facultative anaerobes can grow in the presence or absence of oxygen, showing growth throughout the broth but more concentrated at the top, unlike obligate aerobes which only grow at the top.

Aerotolerant anaerobes have a growth pattern that is evenly distributed throughout the nutrient broth, indicating they do not require oxygen but can tolerate it.

Strict anaerobes show growth only at the bottom of the nutrient broth, as they cannot tolerate oxygen.

Microaerophiles show growth concentrated in the middle of the nutrient broth, as they require lower levels of oxygen than what is present in the atmosphere.

Gram-negative rods appear as vibrant magenta-colored bacteria under a microscope. They vary in length and orientation, with some being isolated and others clustered together. The background is typically light blue, providing a clear contrast to the stained bacteria.

Enterobacteriaceae exhibit good growth on MacConkey agar, producing colonies that are typically pinkish-red due to lactose fermentation.

E. coli colonies on MacConkey agar appear pinkish-red and are densely packed, while Proteus colonies are beige, circular, and less dense.

Gram-positive organisms, such as Staphylococcus, show poorer growth on MacConkey agar, resulting in smaller colonies and sparse growth compared to Enterobacteriaceae.

• Oxidase: Negative (–ve)
• Catalase: Positive (+ve)

A positive result in the fermentative test indicates that Enterobacteriaceae can ferment glucose.

A positive nitrate reductase test indicates that Enterobacteriaceae can reduce nitrate to nitrite, confirming the presence of nitrate reductase activity.

Enterobacteriaceae can be found on plants, in soils, water, and in the intestines of humans and animals.

Enterobacteriaceae are part of the normal flora in the human intestine, except for the pathogenic genera.

1. Primary Pathogens: These are capable of causing diseases in anyone infected, regardless of their health status.

2. Opportunistic Pathogens: Most members of Enterobacteriaceae fall into this category, meaning they typically do not cause severe infections in healthy individuals but can lead to infections in those with weakened immune systems or other risk factors.

The primary pathogens include Salmonella, Shigella, Yersinia, and E. coli.

Gastrointestinal infections are transmitted via the faecal-oral route, primarily through:

1. Contaminated food
2. Poor personal hygiene

Salmonella can be classified based on:

1. O-antigen (cell surface antigen)
2. H-antigen (flagellar antigen)

This classification is often performed using an agglutination test with standard antisera.

The serological classification of Salmonella, such as the Kauffman & White Scheme, is used for:

• Epidemiological tracing of the source of infections.

Humans are the only primary reservoir of Shigella.

The four species of Shigella are:

1. Sh. dysenteriae
2. Sh. flexneri
3. Sh. boydii
4. Sh. sonnei

Shigella dysenteriae causes dysentery, characterized by blood-stained diarrhoea.

The other three species (Sh. flexneri, Sh. boydii, and Sh. sonnei) cause diarrhoea.

Yersinia enterocolitica is the primary pathogenic species responsible for gastro-intestinal infections.

Yersinia pestis is a dangerous pathogen that causes the plague, also known as the 'Black Death', and is transmitted by flea bites.

Yersinia pestis produces plasmin, which dissolves blood clots, facilitating its spread within the host.

• Lymph node enlargement
• Fever
• Painful joints

• Transmitted via aerosol
• Symptoms include fever, cough, and blood-stained sputum. It is the most contagious form of plague.

• It is the least common form of plague.
• Symptoms include fevers and purple skin patches due to disseminated intravascular coagulation.

E. coli colonies appear as pink/magenta on MacConkey agar.

The pathogenic strains of E. coli include:

1. Enteropathogenic E. coli
2. Enterohaemorrhagic E. coli (0157:H7)
3. Enterotoxigenic E. coli
4. Enteroinvasive E. coli
5. Enteroaggregative E. coli

Enteropathogenic E. coli damages the intestinal epithelium primarily through adhesion. It uses bundle-forming pili (bfp) for initial attachment and causes intimate adhesion, leading to actin condensation and microvilli effacement.

Enterotoxigenic E. coli produces two types of enterotoxins: heat-labile toxin (LT) and heat-stable toxin (ST). It is commonly associated with traveller's diarrhoea.

Enteroinvasive E. coli causes damage through its invasiveness, leading to symptoms such as watery diarrhoea with mucus, which may be bloody. It invades the intestinal cells and can spread laterally to other cells after lysing the phagosome.

Enteroaggregative E. coli adheres to the mucus layer of the intestinal villi and delivers cytotoxin, contributing to its pathogenic effects.

Enterohaemorrhagic E. coli (O157:H7) exhibits intimate adhesion to the intestinal villus, leading to actin condensation and microvilli effacement. It delivers Shiga toxin, which can cause bloody diarrhoea and renal failure.

The recent O104 strain of E. coli damages the intestinal epithelium through adhesion, produces cytotoxin (Shiga toxin), and causes diarrhoea at all ages, potentially leading to bloody diarrhoea and renal failure.

Opportunistic pathogens of the Enterobacteriaceae family, such as Proteus, Klebsiella, Citrobacter, and Enterobacter, can cause infections in individuals with weakened immune systems. These infections can lead to serious health complications due to the patients' inability to effectively fight off the pathogens.

Infection sources include endogenous sources or the hospital environment.

Patients are quickly colonized on the skin or respiratory tract.

Extra-intestinal infections include:

1. Wound infections
2. Respiratory tract infections (RTI)
3. Urinary tract infections (UTI)
4. Septicaemia

Klebsiella spp. colonies are large, mucoid, and viscous.

The most important species of Klebsiella is Klebsiella pneumoniae, which causes UTI, RTI, septicaemia, and is an important nosocomial pathogen.

The two most commonly encountered species of Proteus are P. mirabilis and P. vulgaris.

Proteus species produce swarming colonies on agar, which are characterized by a somewhat irregular pattern.

The most important species of Enterobacter are E. aerogenes and E. cloacae. They are known to cause urinary tract infections (UTI), respiratory tract infections (RTI), septicaemia, and are significant nosocomial pathogens.

The more common species of Citrobacter are C. diversus (indole positive) and C. freundii (H₂S positive).

• Gram-negative rods (GNR)
• Good growth on MacConkey agar
• Oxidase negative (Ox -)
• Nitrate positive (Nitrate +)

Most genera cause opportunistic and healthcare-associated infections, particularly extra-intestinal tract infections.

The pathogenic genera cause gastrointestinal tract infections and are typically non-lactose fermenters, with the exception of pathogenic E. coli.