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Quantitation of mecA Transcription in Oxacillin-Resistant Staphylococcus aureus Clinical Isolates. Adriana E. Rosato, 2003.The transcription of mecA, the gene required for oxacillin resistance in staphylococci, was quantified in a collection of 65 geographically and genetically diverse clinical and 8 defined laboratory Staphylococcus aureus isolates . mecA transcription was measured by real-time reverse transcription-PCR, confirmed by Northern blot analysis, and correlated with the presence and DNA sequence of the two mecA repressors, mecI and blaI . Isolates were first examined that contained mecI and/or blaI with wild-type sequence . BlaI provided significantly more repression of mecA transcription than did MecI, unrelated to blaI genetic location . Both together repressed mecA better than either one alone . In clinical isolates containing only wild-type mecI, mecA transcription repression was 10- to 25-fold less effective than that seen in previously studied constructs derived from strain N315 . There was a difference in the mecI ribosomal binding site (RBS) between the clinical isolates (GGAA) and N315 (GGAG) . The GGAA RBS was associated with 5.5- to 7.3-fold less mecA repression than GGAG in isogenic constructs . The values generated for wild-type repressors were compared to those in 26 isolates containing mecI mutations . mecA transcription appeared to be repressed only by BlaI in isolates with mecI nonsense and frameshift mutations . In contrast, mecI repression seemed to be partially or fully retained in many of the isolates with mecI and one isolate with blaI missense mutations, providing structure-function correlates with the site and type of mutation . We conclude that mecA repressor activity is highly variable in clinical S . aureus isolates due to mecI mutations, RBS polymorphisms, and unidentified genomic adaptations . Ecology and Transmission of Listeria monocytogenes Infecting Ruminants and in the Farm Environment. K. K. Nightingale, 2004.A case-control study involving 24 case farms with at least one recent case of listeriosis and 28 matched control farms with no listeriosis cases was conducted to probe the transmission and ecology of Listeria monocytogenes on farms . A total of 528 fecal, 516 feed, and 1,012 environmental soil and water samples were cultured for L . monocytogenes . While the overall prevalence of L . monocytogenes in cattle case farms (24.4%) was similar to that in control farms (20.2%), small-ruminant (goat and sheep) farms showed a significantly (P < 0.0001) higher prevalence in case farms (32.9%) than in control farms (5.9%) . EcoRI ribotyping of clinical (n = 17) and farm (n = 414) isolates differentiated 51 ribotypes . L . monocytogenes ribotypes isolated from clinical cases and fecal samples were more frequent in environmental than in feed samples, indicating that infected animals may contribute to L . monocytogenes dispersal into the farm environment . Ribotype DUP-1038B was significantly (P < 0.05) associated with fecal samples compared with farm environment and animal feedstuff samples . Ribotype DUP-1045A was significantly (P < 0.05) associated with soil compared to feces and with control farms compared to case farms . Our data indicate that (i) the epidemiology and transmission of L . monocytogenes differ between small-ruminant and cattle farms; (ii) cattle contribute to amplification and dispersal of L . monocytogenes into the farm environment, (iii) the bovine farm ecosystem maintains a high prevalence of L . monocytogenes, including subtypes linked to human listeriosis cases and outbreaks, and (iv) L . monocytogenes subtypes may differ in their abilities to infect animals and to survive in farm environments .
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