Antibiotic resistance of aquatic bacteria and it's implications for limnological research

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The antibiotic resistance of bacteria from the water column of various rivers and billabongs in north-eastern Victoria was studied during 1988-91. Two indices were used as measures of resistance: conventional disc-sensitivity tests with culturable bacteria, and the effect of antibiotics on the uptake of 14C-glucose and 14C-amino acids by native bacterial assemblages. Culturable bacteria were resistant to ampicillin, methicillin and penicillin but generally sensitive to nalidixic acid, neomycin, streptomycin, sulfafurazole and tetracycline. Responses to chloramphenicol, erythromycin, nitrofurantoin and sulfamethizole were variable. Bacteria isolated from rivers were more resistant to antibiotics than were those isolated from billabongs. Methicillin, even at 200 mg L-1, had little effect on the uptake of 14C-labelled compounds. Nalidic acid (20 mg L-1) also was ineffective unless lengthy pre-incubations (24 h) were used. Chloramphenicol and tetracycline at 20 mg L-1 markedly reduced amino acid uptake but not glucose uptake. The widespread antibiotic resistance of aquatic bacteria casts doubt on the suitability of these compounds to selectively inhibit the prokaryotic component in ecological studies and may be cause for concern regarding the spread of antibiotic resistance through natural aquatic communities.