Effects of saliva and alpha-amylase on antibiotic sensitivity of bacteria

Two hundred and ninety-six bacterial isolates were investigated for the effects of saliva and alpha-amylase on their susceptibility to ampicillin, tetracycline, chloramphenicol and gentamicin. When the test organisms were primed with normal and 'diseased' saliva there were no observable differences in the MICs of ampicillin and chloramphenicol for group-A streptococci, but alpha-amylase significantly reduced the MIC of tetracycline from 2 to 0.25 mg/1. With Staphyloccus aureus, priming with saliva and alpha-amylase had no effect on the MICs of gentamicin and ampicillin, whereas the MICs of tetracycline and chloramphenicol were increased. The effect of saliva on the susceptibility of E. coli to tetracycline was also significant; MIC50 and MIC90 were reduced from 128 to 8 and 32 mg/1 respectively. Chloramphenicol was however increased from less than 0.125 to 1 and 2 mg/1 when E. coli was primed with amylase and saliva respectively. The general significance of these observations is discussed.