4. When are bacteria said to be “resistant”?
Although antimicrobial products are used in concentrations that are usually sufficient to inhibit or kill the targeted bacteria, some strains of bacteria are able to survive and even grow at these concentrations; they are said to be “resistant”.
- when a strain is not killed or inhibited by the antimicrobial concentration typically used in practice,
- when a strain is not killed or inhibited by a concentration at which the majority of strains of that micro-organism are affected
- when bacterial cells are not killed or inhibited by a concentration acting upon the majority of cells in that culture.
Bacteria are called “insusceptible” when they have natural (innate) properties, such as a specific envelope structure, that impairs biocide penetration. Bacteria develop “ tolerance ” if they become less affected by a biocide concentration that is active on susceptible strains, so that higher concentrations of the biocide are needed to stop them multiplying.
Bacteria can transfer diverse bits of genetic material (plasmids, transposons, etc.) to other bacteria containing several associated genes. When genetic information coding for different antimicrobial resistance mechanisms is transferred to a new host it is referred to as “co-resistance”.