| Summary: | I
n order to evaluate the impact of pollution and sewage on the occurrence and
antibiotic resistance of mesophilic aeromonads in riverine freshwaters of
Marrakech, samples were collected from three rivers (Oukaimeden, Ourika, and
Tensift) upstream and downstream from the principal bordering villages. During a
2-year study, indicators of pollution increased dramatically in the downstream
waters. Bacterial indicators (faecal coliforms and faecal streptococci) correlated
with mesophilic aeromonads only in heavily polluted waters. In low and moderately
polluted sources, densities of mesophilic aeromonads were independent of
water quality indicators and did not correlate statistically with faecal indicators.
Average counts of Aeromonas in low and heavily polluted waters were 2.5 × 103
and 2.1 × 106 colony forming units per 100 ml, respectively. The biochemical
identification of 841 isolates indicated a predominance of A. caviae in heavily and
moderately polluted water and sediment. A. hydrophila was dominant only in low
polluted waters and when the temperature was below 12°C. High densities of A.
sobria were found in low, moderately polluted, or cleaned waters and when the
water temperature was above 18°C. All selected isolates (total = 841) were tested
for antibiotic susceptibility against 21 antibiotics. Antibiotic resistance frequencies
recorded were: ampicillin and amoxicillin, 100%; novobiocin, 96%; cefalotin,
81%; colistin, 72%; sulfamethoxazole, 40%; cefamandole, 37%; polymyxin B, 23%;
trimethoprim, 17%; erythromycin, 15%; streptomycin, 8%; amoxicillin-clavulanate,
5%. Resistance to cefotaxime, kanamycin, gentamycin, chloramphenicol,
tetracycline, oxytetracycline, nalidixic acid, rifampicin, or trimethoprim-sulfamethoxazole
was found to be <5%. Antibiotic resistance rates did vary according to the
source of a strain’s isolation, and high numbers of antibiotic resistant strains
were recorded in polluted samples. Since no correlation between mesophilic
aeromonads and conventional faecal pollution indicators was observed in low or
moderately polluted waters, and since these freshwaters are used for domestic
supply, we propose the use of mesophilic aeromonads as complementary water
pollution indicators to ensure the safety of water.
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