Question

1. Using a textbook and a reputable online source (such as the CDC), research and describe some potentially pathogenic members of the intestinal bacteria family Enterobacteriaceae. Which pathogenic species are lactose fermenters that will grow on MacConkey agar?

Answer 1

Enterobacteriaceae is a large family of Gram-negative bacteria.

Members of the Enterobacteriaceae are often referred to as enterics because the principal habitat of many of these organisms is the lower gastrointestinal tract of various animals.

So some of the potential enteric pathogens are:-

Enterobacter-These genera can be highly antimicrobial resistant.

E.coli- The majority of E. coli in the GI tract are harmless unless they are displaced to other parts of the body such as the urinary tract or meninges. The pathogenic, or enterovirulent E. coli are divided into 4 groups according to their action in the body

Enteropathogenic (EPEC)

Enterotoxigenic (ETEC)

Enteroinvasive (EIEC)

Enterohaemorrhagic (EHEC)

Citrobacter- Citrobacter freundii and C. diversus (C koseri) have been isolated predominantly as superinfecting agents from urinary and respiratory tract infections.

Citrobacter species also cause meningitis, septicemia, and pulmonary infections in neonates and young children.

Klebsiella- Klebsiella causes a severe pneumonia; K rhinoscleromatis causes rhinoscleroma; and K ozaenae is associated with ozena, an atrophic disease of the nasal mucosa.

Salmonella- Salmonella is a rod-shaped gram-negative motile bacterium. There is widespread occurrence in animals, especially in poultry and swine.

Salmonella gastroenteritis usually follows the ingestion of food or drinking water contaminated by faces and accounts for 15% of foodborne infection. Enteric fever is caused by S. typhi.

Shigella- The Shigella species are the principal agents of bacterial dysentery. The disease consists of fever, abdominal cramps, and bloody diarrhoea with mucus.

The genus Shigella and Escherichia are much closer in DNA homology than other species.

Proteus- It is a group of nosocomial pathogen responsible for 46% of urinary tract and 24% of surgical site infections, 17% of the bacteremias, and 30% of the pneumonias. Proteus mirabilis is the most frequent cause of infection-related kidney stones.

All of these intestinal pathogens are Gram negative, facultative anaerobic, non-spore- forming rods that are typically motile, except for Klebsiella and Shigella which is nonmotile.

Virulence often depends on the presence of attachment pili. The major classes of antigens used in defining strains are H (flagellar), O (somatic), and K(capsular). Enteropathogens have diverse mechanisms for disease production which include different toxins and colonization factors.

MacConkey agar was developed in 20th century by Alfred Theodore MacConkey. It was the first formulated solid differential media.

MacConkey agar is a selective and differential culture media commonly used for the isolation of enteric Gram-negative bacteria.

Gram-negative enteric bacteria that grow on MacConkey agar are differentiated by their ability to ferment lactose. If the lactose is fermented by the bacteria, the production of the acid drops the pH of the media. The drop in pH is indicated by the change of neutral red indicator to pink (Neutral read appears pink at pH’s below 6.8).

Pathogenic species of Enterobacteriaceae which are lactose fermenters that will grow on MacConckey agar are:-

Lactose usually is fermented rapidly by-

• Escherichia
• Klebsiella
• Some Enterobacter species like Enterobacter aerogenes.

These strong lactose fermenting bacteria produce sufficient acid which causes precipitation of the bile salts around the growth. It appears as a pink halo surrounding individual colonies or areas of confluent growth.

Lactose fermented slowly by-

• Citrobacter and
• Some Serratia species.

Pink halo is not seen around the colonies of these weaker lactose fermenting bacteria.

Gram-negative bacteria that grow on MacConkey agar but do not ferment lactose appear colorless on the medium and the agar surrounding the bacteria remains relatively transparent.