Reading of inhibition zones

E.coli resistant to ampicillin

When discussing the susceptibility of E. coli (or any bacteria) to an antibiotic like ampicillin, laboratory tests are used to categorize bacterial strains into different groups based on their ability to grow in the presence of the antibiotic. These categories— susceptible, resistant and intermediate susceptible — indicate how effective the antibiotic is at inhibiting bacterial growth.

A strain of E. coli that is classified as resistant is not inhibited or killed by ampicillin, even at concentrations that would typically be effective.
Implication: Ampicillin will not be effective in treating infections caused by this strain, and alternative antibiotics should be used.

A strain of E. coli that is classified as susceptible is killed or inhibited by ampicillin at normal, clinically achievable concentrations. In other words, the antibiotic is effective at treating infections caused by this strain.
Implication: Ampicillin can be used to treat infections caused by this strain because the concentration of the drug that can be achieved in the body (e.g., through oral or intravenous dosing) will be sufficient to kill or inhibit the bacteria.

A strain of E. coli classified as intermediate means that the strain is inhibited by ampicillin, but at higher-than-normal concentrations. In other words, the antibiotic may still work, but the strain is more likely to survive in areas where the antibiotic concentration is lower (e.g., due to poor tissue penetration, biofilm formation, or inadequate drug concentration at the infection site). EUCAST does not define the intermediate range of the inhibition zone for ampicillin, although it does for co-trimoxazole (>14 mm to >11 mm) and ciprofloxacin (>25 mm to >22 mm).

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