Antimicrobial drug residues in milk : effect on mesophilic starter cultures

Sammanfattning: Starter cultures have an essential role in cheese production contributing to desired acid production and formation of aroma- and flavour compounds during cheese maturation. Residues of antimicrobial drugs might reduce the activity of dairy starter cultures and subsequently cause impaired cheese quality. The incidence of prescription of antimicrobial drugs to dairy cows in Sweden is low. In addition, the maximum residue levels (MRL) of antimicrobial drugs in bovine milk are regulated. However, studies suggest that residues of antimicrobial drugs might impact on starter culture bacteria even at concentrations below the MRL. The aim of this study was to investigate how penicillin G, oxytetracycline and trimethoprim-sulfadiazine affect the bacteria in commercial mesophilic starter cultures at concentrations corresponding to their respective MRL values. The effect of the individual antimicrobial drugs on the activity of the starter cultures was investigated by measuring pH during fermentation. In addition, changes in starter culture composition was analysed by selective cultivation on x-gal calcium citrate agar. Oxytetracycline at 100 ppb had a significant effect on the fermentation in 2 out of 3 starter cultures, while no significant effect on the culture activity was observed at 4 ppb penicillin G or 100 ppb trimethoprim-sulfadiazine. Penicillin G at 4 ppb had a significant effect on numbers Lactococcus lactis spp. lactis/cremoris in 1 out of 3 starter cultures while no significant effect was observed for Lactococcus lactis spp. lactis biovar. diacetylactis or Leuconostoc spp. Oxytetracycline at 100 ppb had a significant effect on Lactococcus lactis spp. lactis biovar. diacetylactis in 2 out of 3 starter cultures, whereas Leuconostoc spp. and Lactococcus lactis spp. lactis/cremoris were significantly inhibited in 1 out of 3 starter cultures. 100 ppb trimethoprim-sulfadiazine had no clear effect on starter culture bacteria. Further studies are necessary to confirm the results of the study.