Staphylococcus aureus response and adaptation to vancomycin

Antibiotic resistance is an increasing challenge for the human pathogen Staphylococcus aureus. Methicillin-resistant S. aureus (MRSA) clones have spread globally, and a growing number display decreased susceptibility to vancomycin, the favoured antibiotic for treatment of MRSA infections. These vancomycin-intermediate S. aureus (VISA) or heterogeneous vancomycin-intermediate S. aureus (hVISA) strains arise from accumulation of a variety of point mutations, leading to cell wall thickening and reduced vancomycin binding to the cell wall building block, Lipid II, at the septum. They display only minor changes in vancomycin susceptibility, with varying tolerance between cells in a population, and therefore, they can be difficult to detect. In this review, we summarize current knowledge of VISA and hVISA. We discuss the role of genetic strain background or epistasis for VISA development and the possibility of strains being 'transient' VISA with gene expression changes mediated by, for example, VraTSR, GraXSR, or WalRK signal transduction systems, leading to temporary vancomycin tolerance. Additionally, we address collateral susceptibility to other antibiotics than vancomycin. Specifically, we estimate how mutations in rpoB, encoding the β-subunit of the RNA polymerase, affect overall protein structure and compare changes with rifampicin resistance. Ultimately, such in-depth analysis of VISA and hVISA strains in terms of genetic and transcriptional changes, as well as changes in protein structures, may pave the way for improved detection and guide antibiotic therapy by revealing strains at risk of VISA development. Such tools will be valuable for keeping vancomycin an asset also in the future.

Stress resistant rpsU variants of Listeria monocytogenes can become underrepresented due to enrichment bias

Population heterogeneity is an important component of the survival mechanism of Listeria monocytogenes, leading to cells in a population with diverse stress resistance levels. We previously demonstrated that several ribosomal gene rpsU mutations enhanced the stress resistance of L. monocytogenes and lowered the growth rate at 30 °C and lower temperatures. This study investigated whether these switches in phenotypes could result in a bias in strain detection when standard enrichment-based procedures are applied to a variety of strains. Detailed growth kinetics analysis of L. monocytogenes strains were performed, including the LO28 wild type (WT) and rpsU variants V14 and V15, during two commonly used enrichment-based procedures described in the ISO 11290-1:2017 and the U.S. Food and Drug Administration Bacteriological Analytical Manual (BAM). WT had a higher growth rate than the variants during the enrichment processes. Co-culture growth kinetics predictions for WT and rpsU variants showed that the detection chances of the rpsU mutants were reduced from ∼52 % to less than ∼13 % and ∼ 3 % during ISO and BAM enrichment, respectively, which were further validated through subsequent qPCR experiments. Higher heat stress resistance of rpsU variants did not lead to faster recovery during enrichment after heat treatment, and different pre-culturing temperatures before heat treatment did not significantly affect the growth kinetics of the WT and rpsU variants. Additionally, post-enrichment isolation procedures involving streaking on selective agar plates did not show preferences for isolating WT or rpsU variants nor affect the detection chance of rpsU variants. The difference in detection chance suggests that the selective enrichment procedures inadequately represent the genotypic diversity present in a sample. Hence, the enrichment bias during the L. monocytogenes isolation procedure may contribute to the observed underrepresentation of the rpsU mutation among L. monocytogenes isolates deposited in publicly available genome databases. The underrepresentation of rpsU mutants in our findings suggests that biases introduced by standard isolation and enrichment procedures could inadvertently skew our understanding of genetic diversity when relying on public databases.

High prevalence of heteroresistance in Staphylococcus aureus is caused by a multitude of mutations in core genes

Heteroresistance (HR) is an enigmatic phenotype where, in a main population of susceptible cells, small subpopulations of resistant cells exist. This is a cause for concern, as this small subpopulation is difficult to detect by standard antibiotic susceptibility tests, and upon antibiotic exposure the resistant subpopulation may increase in frequency and potentially lead to treatment complications or failure. Here, we determined the prevalence and mechanisms of HR for 40 clinical Staphylococcus aureus isolates, against 6 clinically important antibiotics: daptomycin, gentamicin, linezolid, oxacillin, teicoplanin, and vancomycin. High frequencies of HR were observed for gentamicin (69.2%), oxacillin (27%), daptomycin (25.6%), and teicoplanin (15.4%) while none of the isolates showed HR toward linezolid or vancomycin. Point mutations in various chromosomal core genes, including those involved in membrane and peptidoglycan/teichoic acid biosynthesis and transport, tRNA charging, menaquinone and chorismite biosynthesis and cyclic-di-AMP biosynthesis, were the mechanisms responsible for generating the resistant subpopulations. This finding is in contrast to gram-negative bacteria, where increased copy number of bona fide resistance genes via tandem gene amplification is the most prevalent mechanism. This difference can be explained by the observation that S. aureus has a low content of resistance genes and absence of the repeat sequences that allow tandem gene amplification of these genes as compared to gram-negative species.

A Ribosomal Protein Homolog Governs Gene Expression and Virulence in a Bacterial Pathogen

The molecular machine necessary for protein synthesis, the ribosome, is generally considered constitutively functioning and lacking any inherent regulatory capacity. Yet ribosomes are commonly heterogeneous in composition and the impact of ribosome heterogeneity on translation is not well understood. Here, we determined that changes in ribosome protein composition govern gene expression in the intracellular bacterial pathogen Francisella tularensis. F. tularensis encodes three distinct homologs for bS21, a ribosomal protein involved in translation initiation, and analysis of purified F. tularensis ribosomes revealed they are heterogeneous with respect to bS21. The loss of one homolog, bS21-2, resulted in significant changes to the cellular proteome unlinked to changes in the transcriptome. Among the reduced proteins were components of the type VI secretion system (T6SS), an essential virulence factor encoded by the Francisella Pathogenicity Island. Furthermore, loss of bS21-2 led to an intramacrophage growth defect. Although multiple bS21 homologs complemented the loss of bS21-2 with respect to T6SS protein abundance, bS21-2 was uniquely necessary for robust intramacrophage growth, suggesting bS21-2 modulates additional virulence gene(s) distinct from the T6SS. Our results indicate that ribosome composition in F. tularensis, either directly or indirectly, posttranscriptionally modulates gene expression and virulence. Our findings are consistent with a model in which bS21 homologs function as posttranscriptional regulators, allowing preferential translation of specific subsets of mRNAs, likely at the stage of translation initiation. This work also raises the possibility that bS21 in other organisms may function similarly and that ribosome heterogeneity may permit many bacteria to posttranscriptionally regulate gene expression. IMPORTANCE While bacterial ribosomes are commonly heterogeneous in composition (e.g., incorporating different homologs for a ribosomal protein), how heterogeneity impacts translation is unclear. We found that the intracellular human pathogen Francisella tularensis has heterogeneous ribosomes, incorporating one of three homologs for ribosomal protein bS21. Furthermore, one bS21 homolog posttranscriptionally governs the expression of the F. tularensis type VI secretion system, an essential virulence factor. This bS21 homolog is also uniquely important for robust intracellular growth. Our data support a model in which bS21 heterogeneity leads to modulation of translation, providing another source of posttranscriptional gene regulation. Regulation of translation by bS21, or other sources of ribosomal heterogeneity, may be a conserved mechanism to control gene expression across the bacterial phylogeny.

MRSA in Humans, Pets and Livestock in Portugal: Where We Came from and Where We Are Going

Over the years, molecular typing of methicillin-resistant S. aureus (MRSA) has allowed for the identification of endemic MRSA strains and pathogenic strains. After reaching a peak of predominance in a given geographic region, MRSA strains are usually replaced by a new strain. This process is called clonal replacement and is observed worldwide. The worldwide spread of hospital-associated MRSA (HA-MRSA), community-associated MRSA (CA-MRSA) and livestock-associated MRSA (LA-MRSA) clones over the last few decades has allowed this microorganism to be currently considered a pandemic. In Portugal, most HA-MRSA infections are associated with EMRSA-15 (S22-IV), New York/Japan (ST5-II) and Iberian (ST247-I) clones. Regarding the strains found in the community, many of them are frequently associated with the hospital environment, namely the Pediatric, Brazilian and Iberian clones. On the other hand, a strain that is typically found in animals, MRSA clonal complex (CC) 398, has been described in humans as colonizing and causing infections. The ST398 clone is found across all animal species, particularly in farm animals where the economic impact of LA-MRSA infections can have disastrous consequences for industries. In contrast, the EMRSA-15 clone seems to be more related to companion animals. The objective of this review is to better understand the MRSA epidemiology because it is, undoubtedly, an important public health concern that requires more attention, in order to achieve an effective response in all sectors.

Reduced Vancomycin Susceptibility, MRSA and Treatment Failure in Pediatric Staphylococcus aureus Bloodstream Infections

BACKGROUND

Clinical implications of reduced vancomycin susceptibility (RVS) among pediatric Staphylococcus aureus bloodstream infections are unknown.

METHODS

We identified all children at 2 children's hospitals with ≥1 blood culture positive for S. aureus. We compared patient and clinical factors for RVS and non-RVS infections using Wilcoxon rank-sum and chi-squared tests. Treatment failure and the duration of bacteremia for RVS versus non-RVS and for methicillin-resistant Staphylococcus aureus (MRSA) versus methicillin-susceptible Staphylococcus aureus (MSSA) infections were compared using multivariable logistic and Poisson regressions, respectively. For MRSA infections, the association of empiric vancomycin monotherapy with treatment failure was assessed using multivariable logistic regression.

RESULTS

RVS was present in 72% (309/426) of cases. No patient or infection characteristics, including methicillin resistance, were associated with RVS. RVS was associated with an increased duration of bacteremia compared with non-RVS infections, aIRR = 1.15 (95% confidence interval: 1.02-1.30). The odds of treatment failure was similar for RVS and non-RVS infections, aOR = 1.04 (0.62-1.74). In contrast, MRSA infections were more likely to have treatment failure than MSSA infections, aOR = 3.03 (95% confidence interval: 1.84-5.00). For MRSA infections, empiric vancomycin monotherapy was associated with an increased odds of treatment failure compared with non-vancomycin or combination anti-MRSA antibiotics, aOR = 3.23 (1.12-9.26).

CONCLUSIONS

RVS was common and was associated with a longer duration of bacteremia but not with treatment failure. Treatment failure was more common for MRSA than for MSSA bloodstream infections. Empiric vancomycin monotherapy increased the odds of treatment failure for MRSA infections.

Discerning the role of polymicrobial biofilms in the ascent, prevalence, and extent of heteroresistance in clinical practice

Antimicrobial therapy is facing a worrisome and underappreciated challenge, the phenomenon of heteroresistance (HR). HR has been gradually documented in clinically relevant pathogens (e.g. Pseudomonas aeruginosa, Staphylococcus aureus, Burkholderia spp., Acinetobacter baumannii, Klebsiella pneumoniae, Candida spp.) towards several drugs and is believed to complicate the clinical picture of chronic infections. This type of infections are typically mediated by polymicrobial biofilms, wherein microorganisms inherently display a wide range of physiological states, distinct metabolic pathways, diverging refractory levels of stress responses, and a complex network of chemical signals exchange. This review aims to provide an overview on the relevance, prevalence, and implications of HR in clinical settings. Firstly, related terminologies (e.g. resistance, tolerance, persistence), sometimes misunderstood and overlapped, were clarified. Factors generating misleading HR definitions were also uncovered. Secondly, the recent HR incidences reported in clinically relevant pathogens towards different antimicrobials were annotated. The potential mechanisms underlying such occurrences were further elucidated. Finally, the link between HR and biofilms was discussed. The focus was to recognize the presence of heterogeneous levels of resistance within most biofilms, as well as the relevance of polymicrobial biofilms in chronic infectious diseases and their role in resistance spreading. These topics were subject of a critical appraisal, gaining insights into the ascending clinical implications of HR in antimicrobial resistance spreading, which could ultimately help designing effective therapeutic options.