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Binding of Staphylococcus aureus to milk fat globules increases resistance to penicillin-G
T Ali-Vehmas1, P Westphalen, V Myllys
1Faculty of Veterinary Medicine, Pharmacology and Toxicology, Helsinki University, Finland.
Abstract:
The susceptibility to penicillin-G of Staphylococcus aureus strains that cause mastitis was tested in milk and in Iso-sensitest broth (ISB): The minimal inhibitory concentrations (MIC) of beta-lactamase-positive strains in milk were 10-100-fold those in ISB, whereas the MIC of beta-lactamase-negative strains in milk were some 10-fold those in ISB; beta-lactamase production was induced by milk in beta-lactamase-positive strains. Much of the increase in resistance to penicillin-G caused by milk can be attributed to milk fat globules; the increase in resistance was related to the binding capacity of the bacteria to milk fat globules as well as to capsule formation by the bacteria. It appears that 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 growth in artificial broth medium in which antibiotic susceptibility testing is usually carried out.
Insights
Bovine mastitis-causing Staphylococcus aureus strains show increased penicillin-G resistance in milk due to milk fat globule binding and capsule formation. This biofilm-like growth alters antibiotic susceptibility compared to standard broth testing.
Area of Science:
- Veterinary Microbiology
- Bacterial Pathogenesis
- Antimicrobial Resistance
Background:
- Staphylococcus aureus is a primary cause of bovine mastitis.
- Standard antibiotic susceptibility testing (AST) in broth may not accurately reflect in-situ bacterial behavior.
- Milk components can influence bacterial physiology and drug response.
Purpose of the Study:
- To investigate the impact of milk environment on Staphylococcus aureus susceptibility to penicillin-G.
- To elucidate the mechanisms behind altered antibiotic resistance in milk.
- To compare bacterial behavior in milk versus standard broth media.
Main Methods:
- Minimal inhibitory concentrations (MICs) of penicillin-G were determined for Staphylococcus aureus strains in both milk and Iso-sensitest broth (ISB).
- Beta-lactamase production was assessed and induced by milk.
- Bacterial adherence to milk fat globules and capsule formation were evaluated.
Main Results:
- Penicillin-G MICs were 10-100-fold higher in milk for beta-lactamase-positive strains and 10-fold higher for beta-lactamase-negative strains compared to ISB.
- Milk induced beta-lactamase production in susceptible strains.
- Increased resistance correlated with bacterial binding to milk fat globules and capsule formation, suggesting biofilm development.
Conclusions:
- The milk environment, particularly milk fat globules, significantly increases Staphylococcus aureus resistance to penicillin-G.
- Biofilm-like growth on milk fat globules alters bacterial response to antibiotics, diverging from planktonic growth observed in broth.
- Standard AST methods may underestimate penicillin-G resistance in mastitis-causing Staphylococcus aureus.