There are three types of resistance:

• Inherent
• Chromosomal
• Plasmid-mediated.

Inherent resistance
An inherent feature of an organism to a particular antimicrobial or group of antimicrobials. Examples include:

• Anaerobic organisms and aminoglycosides. The aminoglycosides require an oxygen-dependent carrier to enter the bacterial cell
• Mycoplasmal organisms and beta-lactams. Mycoplasmas have no cell wall, which is the target for beta-lactam antimicrobials.

Chromosomal resistance

• 1:10,000,000 bacterial cells give rise to a daughter cell with a mutation.
• This may lead to a change which confers resistance on that organism.
• This type of resistance develops slowly and often requires multiple steps.

Plasmid-mediated resistance

• Plasmids are small circular pieces of DNA carried by many bacterial cells.
• They may be transferred between bacteria –
• even between different bacterial species, e.g. from a non-pathogenic to a pathogenic organism.
• They may carry genes which impart resistance to several different types of antimicrobial.

Thus, in a single step an organism may become resistant to multiple antimicrobials.