Healthcare-associated carbapenem-resistant Acinetobacter baumannii (CRAB) infections are a serious threat associated with global epidemic clones and a variety of carbapenemase gene classes. In this study, we describe the molecular epidemiology, including whole-genome sequencing analysis and antimicrobial susceptibility profiles of 92 selected, nonredundant CRAB collected through public health efforts in the United States from 2013 to 2017. Among the 92 isolates, the Oxford (OX) multilocus sequence typing scheme identified 30 sequence types (STs); the majority of isolates (n = 59, 64%) represented STs belonging to the international clonal complex 92 (CC92(OX)). Among these, ST208(OX) (n = 21) and ST281(OX) (n = 20) were the most common. All isolates carried an OXA-type carbapenemase gene, comprising 20 alleles. Ninety isolates (98%) encoded an intrinsic OXA-51-like enzyme; 67 (73%) harbored an additional acquired bla(OXA) gene, most commonly bla(OXA-23) (n = 45; 49%). Compared with isolates harboring only intrinsic oxacillinase genes, acquired bla(OXA) gene presence was associated with higher prevalence of resistance and a higher median minimum inhibitory concentration to the carbapenem imipenem (64 μg/mL vs. 8 μg/mL), and antibiotics from other drug classes, including penicillin, aminoglycosides, cephalosporins, and polymyxins. These data illustrate the wide distribution of CC92(OX) and high prevalence of acquired bla(OXA) carbapenemase genes among CRAB in the United States.

USA500 isolates are clonal complex 8 (CC8) Staphylococcus aureus strains closely related to the prominent community- and hospital-associated USA300 group. Despite being relatively understudied, USA500 strains cause a significant burden of disease and are the third most common methicillin-resistant S. aureus (MRSA) strains identified in the U.S. Emerging Infections Program (EIP) invasive S. aureus surveillance. To better understand the genetic relationships of the strains, we sequenced the genomes of 539 USA500 MRSA isolates from sterile site infections collected through the EIP between 2005 and 2013 in the United States. USA500 isolates fell into three major clades principally separated by their distribution across different U.S. regions. Clade C1 strains, found principally in the Northeast, were associated with multiple IS256 insertion elements in their genomes and higher levels of antibiotic resistance. C2 was associated with Southern states, and E1 was associated with Western states. C1 and C2 strains all shared a frameshift in the gene encoding AdsA surface-attached surface protein. We propose that the term "USA500" should be used for CC8 strains sharing a recent common ancestor with the C1, C2, and E1 strains but not in the USA300 group.IMPORTANCE In this work, we have removed some of the confusion surrounding the use of the name "USA500," placed USA500 strains in the context of the CC8 group, and developed a strategy for assignment to subclades based on genome sequence. Our new phylogeny of USA300/USA500 will be a reference point for understanding the genetic adaptations that have allowed multiple highly virulent clonal strains to emerge from within CC8 over the past 50 years.

INTRODUCTION: Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) is a prevalent cause of skin and soft tissue infections (SSTI), but the association between CA-MRSA colonization and infection remains uncertain. We studied the carriage frequency at several body sites and the diversity of S. aureus strains from patients with and without SSTI. MATERIALS AND METHODS: Case-subjects with a closed skin abscess (i.e., without drainage) and matched control subjects without a skin infection (N=147 each) presenting to 10 U.S. emergency departments were cultured at the nares, throat, rectum, and groin using broth enrichment; wounds were cultured from abscess cases. Methicillin resistance testing and spa typing were performed for all S. aureus isolates. RESULTS: S. aureus was found in 85/147 (57.8%) of abscesses; 49 were MRSA, 36 were MSSA. MRSA colonization was more common among cases (59/147; 40.1%) than controls (27/147; 18.4%) overall (p<0.001), and at each body site; no differences were observed for MSSA. S. aureus-infected subjects were usually (75/85) colonized with the infecting strain; among MRSA-infected subjects this was most common in the groin. The CC8 lineage accounted for most of both infecting and colonizing isolates, although more than 16 distinct strains were identified. Nearly all MRSA infections were inferred as USA300. There was more diversity among colonizing than infecting isolates, and among those isolated from controls versus cases. CONCLUSIONS: CC8 S. aureus is a common colonizer of persons with and without skin infections. Detection of S. aureus colonization, and especially MRSA, may be enhanced by extra-nasal site culture.

We assessed for vancomycin-resistant Staphylococcus aureus (VRSA) precursor organisms in southeastern Michigan, an area known to have VRSA. The prevalence was 2.5% (pSK41-positive methicillin-resistant S. aureus, 2009-2011) and 1.5% (Inc18-positive vancomycin-resistant Enterococcus, 2006-2013); Inc18 prevalence significantly decreased after 2009 (3.7% to 0.82%). Risk factors for pSK41 included intravenous vancomycin exposure.

Vancomycin-resistant Staphylococcus aureus (VRSA), important multidrug-resistant organisms of public health concern, have been infrequently identified in the United States since 2002. All previous VRSA belonged to clonal complex 5, a lineage associated primarily with healthcare. This report describes the most recent (13th) U.S. VRSA; the first in a community strain.

BACKGROUND: Of the 13 US vancomycin-resistant Staphylococcus aureus (VRSA) cases, 8 were identified in southeastern Michigan, primarily in patients with chronic lower-extremity wounds. VRSA infections develop when the vanA gene from vancomycin-resistant enterococcus (VRE) transfers to S. aureus. Inc18-like plasmids in VRE and pSK41-like plasmids in S. aureus appear to be important precursors to this transfer. OBJECTIVE: Identify the prevalence of VRSA precursor organisms. DESIGN: Prospective cohort with embedded case-control study. PARTICIPANTS: Southeastern Michigan adults with chronic lower-extremity wounds. METHODS: Adults presenting to 3 southeastern Michigan medical centers during the period February 15 through March 4, 2011, with chronic lower-extremity wounds had wound, nares, and perirectal swab specimens cultured for S. aureus and VRE, which were tested for pSK41-like and Inc18-like plasmids by polymerase chain reaction. We interviewed participants and reviewed clinical records. Risk factors for pSK41-positive S. aureus were assessed among all study participants (cohort analysis) and among only S. aureus-colonized participants (case-control analysis). RESULTS: Of 179 participants with wound cultures, 26% were colonized with methicillin-susceptible S. aureus, 27% were colonized with methicillin-resistant S. aureus, and 4% were colonized with VRE, although only 17% consented to perirectal culture. Six participants (3%) had pSK41-positive S. aureus, and none had Inc18-positive VRE. Having chronic wounds for over 2 years was associated with pSK41-positive S. aureus colonization in both analyses. CONCLUSIONS: Colonization with VRSA precursor organisms was rare. Having long-standing chronic wounds was a risk factor for pSK41-positive S. aureus colonization. Additional investigation into the prevalence of VRSA precursors among a larger cohort of patients is warranted.

We compared recovery of Staphylococcus aureus and methicillin-resistant S. aureus (MRSA) from nasal and groin swabs of 600 HIV-infected outpatients by selective and non-selective direct plating and broth enrichment. Swabs were collected at baseline, 6-month and 12-month visits, cultured by direct plating to Mannitol Salt Agar (MSA), CHROMagar MRSA (CM), and overnight broth enrichment with sub-culture to MSA (Broth). MRSA isolates were characterized by pulsed-field gel electrophoresis (PFGE), SCCmec typing and PCR for the Panton-Valentine leukocidin. At each visit 13-15% of patients were colonized with MRSA and 30-33% with methicillin-susceptible S. aureus (MSSA). Broth, CM and MSA detected 95%, 82% and 76% of MRSA-positive specimens, respectively. MRSA recovery was significantly higher from Broth compared to CM (p ≤ 0.001) or MSA (p ≤ 0.001); there was no significant difference in recovery between MSA and CM. MSSA recovery also increased significantly using Broth compared to MSA (p ≤ 0.001). Among specimens collected from the groin, Broth, CM, and MSA detected 88%, 54%, and 49% of the MRSA-positive isolates, respectively. Broth enrichment had a greater impact on recovery of MRSA from the groin than from the nose compared to both CM (p ≤ 0.001) and MSA (p ≤ 0.001). Overall, 19% of MRSA-colonized patients would have been missed with nasal culture only. USA500/Iberian and USA300 were the most common MRSA strains recovered, and USA300 was more likely than other strain types to be recovered from the groin than from the nose (p=0.05).