Evaluation of the effect of paromomycin dosed at 150 mg/kg on commensal gut microbiota of healthy calves

Twenty-five healthy calves originating from 7 diferent French farms and aged 12-21 days at the start of the study were enroled. Calves were housed colectively, fed with milk replacers twice a day and randomly alocated to treatment (Gabbrovet Multi®, Ceva Santé Animale, 150 mg/kg once a day for 5 days; n=23) or untreated control group (n=2) on Day 0. Calves’ health was monitored daily based on faeces status, depression score, and appetite (0-2 scorings). At Day+37, calves were euthanized. Faecal samples before, during and after treatment with paromomycin were colected to trace the evolution of paromomycin susceptibility of commensal intestinal Escherichia coli. For this, fresh faeces of the animals were colected at Day-1, Day+4, Day+20 and Day+36 in a sterile 50 ml sample container then immediately stored at -80°C and later transported frozen to a microbiology laboratory for isolation and purification of commensal E. coli strains. Twenty purified and randomly isolated colonies of E. coli were recovered from every faecal sample of each colection time point for further microbiological analysis. Because of the very high number of strains, clusters of epidemiologicaly related strains were identified by mean of mass spectrometry (MaldiTof Biotyper Compass explorer software), then subjected to paromomycin MIC determination by mean of a customized microdilution approach (UMIC) and an aminoglycoside antibiogram in accordance with CLSI guidelines. Based on the CA-SFM breakpoint of kanamycin for Enterobacteriaceae, the evolution of resistance to paromomycin of the E. coli strains was monitored for every strain/sample by comparing the results obtained individualy for each animal at diferent times during the study with those obtained prior to paromomycin administration.

During this study, no acquisition of resistance at the clonal level was observed in E. coli folowing paromomycin treatment. The susceptibility profile tracked for each clone of E. coli did not show any shift of the MIC between the sampling points. The treatment as expected disrupted the flora by preferentia ly selecting preexisting resistant clones capable of surviving. A gradual reversion to the initial conditions of the colibacilary flora was observed after paromomycin treatment discontinuation. The curative bactericidal high dosage of paromomycin used in this study has certainly contributed in preserving the strains from acquiring resistance. Particular a tention should be taken when lower dosage regimen is used (i.e. prophylactic usage) as these have been documented for inducing more readily resistance.