BACTERIA

Bacteria

Bacteria

Introduction

This group of organisms includes several that cause primary infections of the human gastrointestinal tract. Thus, they are referred to as enterics (regardless of whether they cause gut disorders). Bacteria that affect the gastrointestinal tract include certain strains of E. coli and Salmonella, all 4 species of Shigella, and Yersinia entercolitica. The rheumatic disease, Reiter's syndrome (associated with HLA-B27), can result from prior exposure to Salmonella, Shigella, or Yersinia. Other organisms that are not members of the Enterobacteriacae, including Campylobacter and Chlamydia, are also causative agents of Reiter's syndrome. Yersina pestis (the cause of "plague") will be considered separately with other zoonotic organisms.

Members of this family are major causes of opportunistic infection (including septicemia, pneumonia, meningitis and urinary tract infections). Examples of genera that cause opportunistic infections are: Citrobacter, Enterobacter, Escherichia, Hafnia, Morganella, Providencia and Serratia. Selection of antibiotic therapy is complex due to the diversity of organisms.(Dowell,2009,22-29)

Some of the organisms additionally cause community-acquired disease in otherwise healthy people. Klebsiella pneumoniae is often involved in respiratory infections. The organism has a prominent capsule aiding pathogenicity . The commonest community acquired ("ascending") urinary tract infection is caused by E. coli. The vast majority of urinary tract infections are ascending, often from fecal contamination. Proteus is another common cause of urinary tract infection; the organism produces a urease that degrades urea producing an alkaline urine.

Identification of some common bacteria : lactose -fermenting Enterobacteriaceae

These are Gram-negative facultative anerobic rods. They lack cytochrome oxidase and are referred to as oxidase negative. They are often isolated from fecal matter on agar containing lactose and a pH indicator. Colonies that ferment lactose will produce sufficient acid to cause a color shift in the indicator (Figure 1). E. coli is a fermenter of lactose, while Shigella, Salmonella and Yersinia are non-fermenters. "Non-pathogenic" strains of E. coli (and other lactose-positive enterics) are often present in normal feces. Since they are difficult to differentiate from "pathogenic" E. coli, lactose-negative colonies are often the only ones identified in feces. All Enterobacteriaceae isolated from other sites (which contain low numbers of bacteria [e.g. urine] or are normally sterile [e.g. blood]) are identified biochemically, for example using the API 20E system. Important serotypes can be differentiated by their O (lipopolysaccharide), H (flagellar) and K (capsular) antigens. However, serotyping is generally not performed in the routine clinical laboratory. (Dowell,2009,22-29)

Enterobacter cloacae has been used in biological control of plant diseases (notably the seed-rotting oomycete Pythium ultimum). Transgenic (aargh!?) E. cloacae have been used to control insect pests on mulberry leaves [J Appl Microbiol 2000 Jan;88(1):90-97]. (Dowell,2009,22-29)

Enterobacter infections are becoming more common in intensive care. They are mainly found where:

Infection control is poor (little hand-washing) - many infections are acquired through cross-transmission [ J Infect Dis 2001 Jul 15;184(2):211-214]. Ready transmission occurs - for example, rapid disinfection of rectal thermometers with 80% alcohol does NOT prevent transmission of E. cloacae.

Substantially compromised patients are present - the very young, the very old, and, especially, those with severe underlying disease, such as neoplasms, or ...