Submitted to: Antimicrobial Chemotherapy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/21/2005
Publication Date: 2/3/2006
Citation: Poole, T.L., Callaway, T.R., Bischoff, K.M., Nisbet, D.J. 2006. Macrolide inactivation gene cluster mphA-mrx-mphR adjacent to a class 1 integron in Aeromonas hydrophila isolated from a diarrhoeic pig in Oklahoma. Journal of Antimicrobial Chemotherapy. 57:31-38. Interpretive Summary: Bacteria that cause disease in humans and animals are becoming increasingly resistant to antibiotics. This has caused public health concern because diseases once thought of little significance are reappearing. Bacteria are not only becoming resistant to one or two antibiotics, they are becoming resistant to many antibiotics; such bacteria are called multi-drug resistant bacteria. Some public health officials blame the food animal industry for the emergence of multi-drug resistant bacteria and downplay the role of human medicine. This paper reports a multi-drug resistant bacterium (Aeromonas hydrophila) that was found in a pig with diarrhea in Oklahoma. This bacterium carries new antibiotic resistance genes never before seen from an environmental isolate, but previously found in human clinical isolates of E. coli bacteria. This may provide evidence that this isolate originally came from a human source. Similar bacteria were originally found in the diapers of children at a daycare center in Houston, Texas.
Technical Abstract: Objective: To characterize a multi-drug resistant Aeromonas hydrophila isolate (CVM861) that possesses a high-level macrolide inactivation gene cluster (mphA-mrx-mphR), previously only reported in E. coli. Methods: PCR, gene sequencing and Southern blotting were used to map the mphA-mrx-mphR gene cluster and flanking elements in CVM861. Conjugation experiments were done to determine if the multi-drug resistance genetic element was mobile. Results: The mphA-mrx-mphR gene cluster mapped downstream of a class 1 integron and upstream of an aph (3’) gene, and was present on a Tn21-like element. The gene order determined by sequencing was: intI1-dhfrXII-orfF-aadA2-qacE-sul1-orf5178-tnpA-mphR-mrx-mphA. Horizontal transmission of high level macrolide resistance occurred from CVM861 to E. coli 47011 at a conjugation frequency of 2.02 × 10**-5 per recipient. Conclusions: An mphA-mrx-mphR gene cluster was present downstream of the In2 integron located on a Tn21-like transposon in an A. hydrophila isolate. Whether this recombination event resulted in the truncation of the orf5 sequence is unknown. The presence of other resistance genes downstream of the mphA-mrx-mphR gene cluster, suggests that multiple recombination events have occurred on this genetic element. This is the first known report of the mphA-mrx-mphR gene cluster carried by A. hydrophila and the first known isolation of this cluster in the United States.