Maintenance Of Resistance Genes Research Articles

Prevalence and risk factors associated with emergence of Rhodococcus equi resistance to macrolides and rifampicin in horse-breeding farms in Kentucky, USA

The combination of a macrolide and rifampicin has been the mainstay of therapy in foals with Rhodococcus equi pneumonia for decades. Recent studies suggest that mass antimicrobial treatment of subclinically affected foals over time has selected for antimicrobial resistance. Our objective was to estimate the prevalence of R. equi strains resistant to macrolides and rifampicin at horse breeding farms in Kentucky. A hundred breeding farms in Kentucky were surveyed and R. equi were cultured from soil samples. Data were analyzed with logistic regression and generalized linear modeling (P < 0.05). Seventy-six percent (76%) of farms yielded resistant R. equi, and resistance to macrolides and rifampicin was associated with their use at farms. The present study is the first to report the prevalence and distribution of resistant isolates in the environment of farms in Kentucky, USA. Collectively, previous reports and the data presented herein provide irrefutable evidence of emerging antimicrobial resistance in R. equi with alarming prevalence. Widespread dissemination and maintenance of resistance genes in the environment where many other pathogenic bacteria exist is a concern for both animal and human health.

Reduction in fitness of flea beetles which are homozygous for an autosomal gene conferring resistance to defences in Barbarea vulgaris

Major resistance genes are present in Danish flea beetle (Phyllotreta nemorum) populations, enabling the beetles to utilize a defended plant, Barbarea vulgaris ssp. arcuata, as a host plant, whereas this plant is unsuitable for beetles lacking the resistance genes. Two lines of beetles carrying a resistance gene have been established which are near-isogenic with a susceptible line. Larval survival of offspring from crosses between flea beetles carrying resistance genes and susceptible beetles, tested in bioassays on the defended B. vulgaris, and sex ratios of the survivors, were consistent with the presence of a dominant, autosomal resistance gene in each of the lines. An attempt to produce pure-breeding lines for the autosomal genes revealed that beetles that are homozygous for the resistance gene suffer a high mortality. This result was repeatable for both lines, and when both resistant males and females were used in the crosses. The high mortality was also independent of the plant (defended B. vulgaris or suitable radish) on which the beetles were reared. The results suggest that the time of death of homozygous resistant beetles is variable. The spread and maintenance of resistance genes in flea beetle populations are discussed.