Multi-site and nasal swabbing for carriage of Staphylococcus aureus: what does a single nose swab predict?

BACKGROUND: Carriage of Staphylococcus aureus is a risk for infections. Targeted decolonization reduces postoperative infections but depends on accurate screening. AIM: To compare detection of S. aureus carriage in healthy individuals between anatomical sites and nurse- versus self-swabbing; also to determine whether a single nasal swab predicted carriage over four weeks. METHODS: Healthy individuals were recruited via general practices. After consent, nurses performed multi-site swabbing (nose, throat, and axilla). Participants performed nasal swabbing twice-weekly for four weeks. Swabs were returned by mail and cultured for S. aureus. All S. aureus isolates underwent spa typing. Persistent carriage in individuals returning more than three self-swabs was defined as culture of S. aureus from all or all but one self-swabs. FINDINGS: In all, 102 individuals underwent multi-site swabbing; S. aureus carriage was detected from at least one site from 40 individuals (39%). There was no difference between nose (29/102, 28%) and throat (28/102, 27%) isolation rates: the combination increased total detection rate by 10%. Ninety-nine patients returned any self-swab, and 96 returned more than three. Nasal carriage detection was not significantly different on nurse or self-swab [28/99 (74%) vs 26/99 (72%); χ2: P=0.75]. Twenty-two out of 25 participants with first self-swab positive were persistent carriers and 69/71 with first self-swab negative were not, giving high positive predictive value (88%), and very high negative predictive value (97%). CONCLUSION: Nasal swabs detected the majority of carriage; throat swabs increased detection by 10%. Self-taken nasal swabs were equivalent to nurse-taken swabs and predicted persistent nasal carriage over four weeks.

Multiple-strain colonization in nasal carriers of Staphylococcus aureus.

Staphylococcus aureus is a commensal that can also cause invasive infection. Reports suggest that nasal cocolonization occurs rarely, but the resources required to sequence multiple colonies have precluded its large-scale investigation. A staged protocol was developed to maximize detection of mixed-spa-type colonization while minimizing laboratory resources using 3,197 S. aureus-positive samples from a longitudinal study of healthy individuals in Oxfordshire, United Kingdom. Initial typing of pooled material from each sample identified a single unambiguous strain in 89.6% of samples. Twelve single-colony isolates were typed from samples producing ambiguous initial results. All samples could be resolved into one or more spa types using the protocol. Cocolonization point prevalence was 3.4 to 5.8% over 24 months of follow-up in 360 recruitment-positives. However, 18% were cocolonized at least once, most only transiently. Cocolonizing spa types were completely unrelated in 56% of samples. Of 272 recruitment-positives returning ≥12 swabs, 166 (61%) carried S. aureus continuously but only 106 (39%) carried the same single spa type without any cocolonization; 31 (11%) switched spa type and 29 (11%) had transient cocarriage. S. aureus colonization is dynamic even in long-term carriers. New unrelated cocolonizing strains could increase invasive disease risk, and ongoing within-host evolution could increase invasive potential, possibilities that future studies should explore.

DNA polymorphism in recombining and non-recombining mating-type-specific loci of the smut fungus Microbotryum.

The population-genetic processes leading to the genetic degeneration of non-recombining regions have mainly been studied in animal and plant sex chromosomes. Here, we report population genetic analysis of the processes in the non-recombining mating-type-specific regions of the smut fungus Microbotryum violaceum. M. violaceum has A1 and A2 mating types, determined by mating-type-specific 'sex chromosomes' that contain 1-2 Mb long non-recombining regions. If genetic degeneration were occurring, then one would expect reduced DNA polymorphism in the non-recombining regions of this fungus. The analysis of DNA diversity among 19 M. violaceum strains, collected across Europe from Silene latifolia flowers, revealed that (i) DNA polymorphism is relatively low in all 20 studied loci (π∼0.15%), (ii) it is not significantly different between the two mating-type-specific chromosomes nor between the non-recombining and recombining regions, (iii) there is substantial population structure in M. violaceum populations, which resembles that of its host species, S. latifolia, and (iv) there is significant linkage disequilibrium, suggesting that widespread selfing in this species results in a reduction of the effective recombination rate across the genome. We hypothesise that selfing-related reduction of recombination across the M. violaceum genome negates the difference in the level of DNA polymorphism between the recombining and non-recombining regions, and may possibly lead to similar levels of genetic degeneration in the mating-type-specific regions of the non-recombining 'sex chromosomes' and elsewhere in the genome.