Humans typically host more than 1.5 kg of microflora in their gastrointestinal tracts some of which are probiotics which aid food digestion and inhibit pathogenic bacterial growth. However, when antibiotics which are intended to target pathogenic bacteria are taken, they could also have an adverse effect on beneficial probiotics. I investigated the effects of the antibiotics gentamicin, penicillin and tetracycline on four different species of Lactobacillus probiotics.
I used the Kirby-Bauer method to test the sensitivity of the probiotics to the four antibiotics. I swabbed eight Lactobacillus-MRS Agar plates with each probiotic and placed five gentamicin, penicillin and tetracycline antibiotic sensitivity disks on them. If the probiotic grew uninhibited, this indicated that it was resistant to the antibiotic. If zones of no bacterial growth appeared around the sensitivity disks the probiotic was sensitive. The larger the zone of inhibition, the more sensitive is the probiotic to the antibiotic;; the smaller the zone, the more resistant is the probiotic.
My results showed that the four Lactobacillus probiotics were each variably affected by two of the three antibiotics. Tetracycline affected all the Lactobacillus species. Penicillin affected all the species except L. reuteri. Gentamicin affected only L. reuteri (to a minor extent). As these were in vitro results, they provide only an indication of what may happen in vivo, in the human body. My experiment has demonstrated differing combinations of resistant probiotics that may be used with antibiotic therapies, where the probiotic may still offer benefit to the host.