BINDING OF STAPHYLOCOCCUS-AUREUS TO MILK-FAT GLOBULES INCREASES RESISTANCE TO PENICILLIN-G

The susceptibility to penicillin-G of Staphylococcus aureus strains th at cause mastitis was tested in milk and in Iso-sensitest broth (ISB). The minimal inhibitory concentrations (MIC) of beta-lactamase-positiv e strains in milk were 10-100-fold those in ISB, whereas the MIC of be ta-lactamase-negative strains in milk were some 10-fold those in ISB; beta-lactamase production was induced by milk in beta-lactamase-positi ve strains. Much of the increase in resistance to penicillin-G caused by milk can be attributed to milk fat globules; the increase in resist ance was related to the binding capacity of the bacteria to milk fat g lobules as well as to capsule formation by the bacteria. It appears th at the binding of the staphylococci to the fat globules and bacterial capsule formation resulted in a biofilm type of growth. In this case, the staphylococci behaved differently from the planktonic type of grow th in artificial broth medium in which antibiotic susceptibility testi ng is usually carried out.