gram-negativemedical dictionary

<microbiology> A common class of bacteria normally found in the gastrointestinal tract that can be responsible for disease in man (sepsis).

Bacteria are considered to be gram-negative because of their characteristic staining properties under the microscope, where they either do not stain or are decolourised by alcohol during Gram's method of staining.

This is a primary characteristic of bacteria that have a cell wall composed of a thin layer of peptidoglycan covered by an outer membrane of lipoprotein and lipopolysaccharide containing endotoxin.

The gram staining characteristics of bacteria have resulted in an important classification system for the identification of bacteria.

See: gram-positive

(06 Oct 1997)