Liquid and Dry Swabs for Culture- and PCR-Based Detection of Colonization with Methicillin-Resistant Staphylococcus aureus during Admission Screening

Rapid detection of methicillin-resistant Staphylococcus aureus (MRSA) colonization status facilitates isolation and decolonization and reduces MRSA infections. Liquid but not dry swabs allow fully automated detection methods. However, the accuracy of culture and polymerase chain reaction (PCR) using liquid and dry swabs has not been analyzed. We compared different swab collection systems for routine nasal–throat MRSA screening in patients admitted to a tertiary care trauma center in Germany. Over 3 consecutive months, dry swabs (month 1), ESwabs (month 2), or MSwabs (month 3) were processed using Cepheid GeneXpert, Roche cobas and BD-MAX™ MRSA tests compared to chromogenic culture. Among 1680 subjects, the MRSA detection rate using PCR methods did not differ significantly between dry swabs, ESwab, and MSwab (6.0%, 6.2%, and 5.3%, respectively). Detection rates using chromogenic culture were 2.9%, 3.9%, and 1.9%, using dry, ESwab, and MSwab, respectively. Using chromogenic culture as the “gold standard”, negative predictive values for the PCR tests ranged from 99.2–100%, and positive predictive values from 33.3–54.8%. Thus, efficient and accurate MRSA screening can be achieved using dry, as well as liquid E- or MSwab, collection systems. Specimen collection using ESwab or MSwab facilitates efficient processing for chromogenic culture in full laboratory automation while also allowing molecular testing in automated PCR systems.

Large hospital outbreak of KPC-2-producing Klebsiella pneumoniae: investigating mortality and the impact of screening for KPC-2 with polymerase chain reaction

BACKGROUND

Multi-drug-resistant Klebsiella pneumoniae carbapenemase (KPC)-2-producing K. pneumoniae are an increasing cause of healthcare-associated infections worldwide.

AIMS

To investigate the impact of clinical infection on mortality, and examine the effect of use of KPC-2-specific polymerase chain reaction (PCR) on the time to contact isolation during an outbreak.

METHODS

Cases were defined as patients clinically infected or colonized with KPC-2-producing K. pneumoniae between June 2010 and July 2012. Cases were described by demographic and health characteristics, and the association between infection and mortality, adjusted for comorbidities and demographic characteristics, was determined using Poisson regression with robust standard errors. A comparison was made between the time to contact isolation with a culture-based method and PCR using Wilcoxon's rank sum test.

FINDINGS

Of 72 cases detected, 17 (24%) had undergone transplantation and 21 (29%) had a malignancy. Overall, 35 (49%) cases were clinically infected, with pneumonia and sepsis being the most common infections. Infection was an independent risk factor for mortality (risk ratio 1.67, 95% confidence interval 0.99-2.82). The median time to contact isolation was 1.5 days (range 0-21 days) using PCR and 5.0 days (range 0-39 days) using culture-based methods (P = 0.003). Intermittent negative tests were observed in 48% (14/29) of cases tested using culture-based methods.

CONCLUSION

KPC-2-producing K. pneumoniae mainly affect severely ill patients. Half of the cases developed clinical infection, associated with increased risk of death. As PCR accelerates isolation and provides the opportunity for preventive measures in colonized cases, its use should be implemented promptly during outbreaks. Further studies are needed to enhance knowledge about KPC detection patterns and to adjust screening guidelines.

Signals for bidirectional nucleocytoplasmic transport in the duck hepatitis B virus capsid protein

Hepadnavirus genome replication involves cytoplasmic and nuclear stages, requiring balanced targeting of cytoplasmic nucleocapsids to the nuclear compartment. In this study, we analyze the signals determining capsid compartmentalization in the duck hepatitis B virus (DHBV) animal model, as this system also allows us to study hepadnavirus infection of cultured primary hepatocytes. Using fusions to the green fluorescent protein as a functional assay, we have identified a nuclear localization signal (NLS) that mediates nuclear pore association of the DHBV nucleocapsid and nuclear import of DHBV core protein (DHBc)-derived polypeptides. The DHBc NLS mapped is unique. It bears homology to repetitive NLS elements previously identified near the carboxy terminus of the capsid protein of hepatitis B virus, the human prototype of the hepadnavirus family, but it maps to a more internal position. In further contrast to the hepatitis B virus core protein NLS, the DHBc NLS is not positioned near phosphorylation target sites that are generally assumed to modulate nucleocytoplasmic transport. In functional assays with a knockout mutant, the DHBc NLS was found to be essential for nuclear pore association of the nucleocapsid. The NLS was found to be also essential for virus production from the full-length DHBV genome in transfected cells and from hepatocytes infected with transcomplemented mutant virus. Finally, the DHBc additionally displayed activity indicative of a nuclear export signal, presumably counterbalancing NLS function in the productive state of the infected cell and thereby preventing nucleoplasmic accumulation of nucleocapsids.

Residues critical for formylglycine formation and/or catalytic activity of arylsulfatase A

Sulfatases contain a unique posttranslational modification in their active site, a formylglycine residue generated from a cysteine or a serine residue. The formylglycine residue is part of a sequence that is highly conserved among sulfatases, suggesting that it might direct the generation of this unique amino acid derivative. In the present study residues 68-86 flanking formylglycine 69 in arylsulfatase A were subjected to an alanine/glycine scanning mutagenesis. The mutants were analyzed for the conversion of cysteine 69 to formylglycine and their kinetic properties. Only cysteine 69 turned out to be essential for formation of the formylglycine residue, while substitution of leucine 68, proline 71, and alanine 74 within the heptapeptide LCTPSRA reduced the formylglycine formation to about 30-50%. Several residues that are part of or directly adjacent to an alpha-helix presenting the formylglycine 69 at the bottom of the active site pocket were found to be critical for catalysis. A surprising outcome of this study was that a number of residues fully or highly conserved between all known eukaryotic and prokaryotic sulfatases turned out to be essential neither for generation of formylglycine nor for catalysis.