Utilization of molecular genotyping and geocoding to improve understanding of the transmission dynamics of recently emerging ST239-MRSA-III in Ohio

Background: ST239-MRSA-III is a bacterial hybrid characterized by rapid clonal diversification and geographic spread. Although it is very common world-wide, infection due to this strain has not been reported in the United States since the 1990’s. The strain is associated with epidemic outbreak potential and displays increased drug resistance and increased capacity to cause serious invasive infections. An emergence of ST239-MRSA-III has been identified recently in Ohio, but the origin and relatedness of these isolates are unknown.  

Objective: To better characterize the molecular heterogeneity and transmission dynamics of ST239-MRSA in Ohio, we performed a variety of molecular genotyping procedures along with geocoding.

Methods: Clinical MRSA isolates collected from the Ohio State Health Network from 01/07 to 01/10 were genotyped using repetitive element polymerase chain reaction (rep-PCR; DiversiLab^TM Biomerieux, NC).  Additional molecular typing was performed on selected ST239 MRSA isolates using spa typing, pulsed-field gel electrophoresis (PFGE), staphylococcal cassette chromosome (SCC) mec typing, dru typing, multilocus sequence typing (MLST), and single nucleotide polymorphism (SNP) typing. ArcGIS 9.2 was utilized to geocode patient addresses.   

Results: Clinical isolates from 77 patients were distributed among 8 different rep-PCR genotypes, 8 different PFGE (Brazilian) subtypes, and 4 different spa types (t037, t363, t421, t631). MLST performed on selected isolates confirmed them as clonal complex 8/239, and all isolates were SCCmec type III.  A single predominant PFGE pattern (A) accounted for 75% (58/77) of the isolates. Dru typing of 64 isolates revealed 12 different dru types, including 10 novel types. SNP typing of 21 isolates showed that they all belonged to haplotype 9 of the Asian clade of ST239. Geocoding revealed the ST239 isolates were distributed throughout Ohio. 

 

Number of isolates	rep-PCR pattern #	spa repeats	spa type	PFGE subtypes
29	6	WGKAOMQ	t037	A
16	53	WGKAOMQ	t037	A
4	6	WGKAOMQ	t037	B
3	19	WGKAOMQ	t037	A
3	42	WGKAOMQ	t037	A
2	53	WGKAOMQ	t037	A
2	6	WKAOMQ	t363	A
2	19	WGKAOMQ	t037	C
2	6	WGKAOMQ	t037	C
2	19	WGKAOMQ	t037	D
2	6	WGKAOMQ	t037	D
2	53	WGKAOMQ	t037	E

Conclusions: Despite the observed molecular heterogeneity and widespread geographic distribution in Ohio, phylogenetic analysis of SNP-defined haplotypes indicates that the emerging Ohio ST239-MRSA-III strains all share recent common ancestry. This is likely the result of a single introduction of the strain into the region, with subsequent dissemination. Since the ST239 strain has epidemic potential, identification of transmission dynamics and prevention of spread is necessary. In this regard, application of social network analysis and temporal-spatial analysis, in combination with molecular surveillance, may further improve understanding of transmission.