Metabolic remodeling by RNA polymerase gene mutations is associated with reduced ß-lactam susceptibility in oxacillin-susceptible MRSA.

The emergence of oxacillin-susceptible methicillin-resistant Staphylococcus aureus (OS-MRSA) has imposed further challenges to the clinical management of MRSA infections. When exposed to ß-lactam antibiotics, these strains can easily acquire reduced ß-lactam susceptibility through chromosomal mutations, including those in RNA polymerase (RNAP) genes such as rpoBC, which may then lead to treatment failure. Despite the increasing prevalence of such strains and the apparent challenges they pose for diagnosis and treatment, there is limited information available on the actual mechanisms underlying such chromosomal mutation-related transitions to reduced ß-lactam susceptibility, as it does not directly associate with the expression of mecA. This study investigated the cellular physiology and metabolism of six missense mutants with reduced oxacillin susceptibility, each carrying respective mutations on RpoBH929P, RpoBQ645H, RpoCG950R, RpoCG498D, RpiAA64E, and FruBA211E, using capillary electrophoresis-mass spectrometry-based metabolomics analysis. Our results showed that rpoBC mutations caused RNAP transcription dysfunction, leading to an intracellular accumulation of ribonucleotides. These mutations also led to the accumulation of UDP-Glc/Gal and UDP-GlcNAc, which are precursors of UTP-associated peptidoglycan and wall teichoic acid. Excessive amounts of building blocks then contributed to the cell wall thickening of mutant strains, as observed in transmission electron microscopy, and ultimately resulted in decreased susceptibility to ß-lactam in OS-MRSA. IMPORTANCE: The emergence of oxacillin-susceptible methicillin-resistant Staphylococcus aureus (OS-MRSA) strains has created new challenges for treating MRSA infections. These strains can become resistant to ß-lactam antibiotics through chromosomal mutations, including those in the RNA polymerase (RNAP) genes such as rpoBC, leading to treatment failure. This study investigated the mechanisms underlying reduced ß-lactam susceptibility in four rpoBC mutants of OS-MRSA. The results showed that rpoBC mutations caused RNAP transcription dysfunction, leading to an intracellular accumulation of ribonucleotides and precursors of peptidoglycan as well as wall teichoic acid. This, in turn, caused thickening of the cell wall and ultimately resulted in decreased susceptibility to ß-lactam in OS-MRSA. These findings provide insights into the mechanisms of antibiotic resistance in OS-MRSA and highlight the importance of continued research in developing effective treatments to combat antibiotic resistance.

Transmission is a Noticeable Cause of Resistance Among Treated Tuberculosis Patients in Shanghai, China.

It is generally believed that drug resistance among treated tuberculosis (TB) patients is as a result of acquired drug resistance due to inappropriate treatment. Previous studies have shown that primary drug resistance caused by transmission also plays a role among treated cases. Differentiating the two types of drug resistance will help in developing appropriate strategies for control of drug resistant tuberculosis. In this study, we tested the hypothesis that drug resistance among treated TB patients is mainly caused by primary resistance rather than acquired resistance. Defining resistance profiles by molecular drug susceptibility test, we used Unit Variable Number Tandem Repeats (VNTR) to genotype and Whole Genome Sequencing (WGS) to confirm the accordance of the first and last Mycobacterium tuberculosis isolates from treated pulmonary TB patients in Shanghai from 2009-2015. Among 81 patients with increasing drug resistance, out of 390 patients enrolled, paired isolates from 59.3% (48/81) had different VNTR patterns indicating primary drug resistance. Our results have demonstrated that primary resistance due to exogenous reinfection is the major cause of drug resistance among treated TB patients in Shanghai; thus, strategies aimed at preventing and interrupting transmission are urgently needed to effectively reduce the epidemic of drug resistant tuberculosis.

Prevalence and transmission of pyrazinamide resistant Mycobacterium tuberculosis in China.

Pyrazinamide (PZA) is an important first-line anti-tuberculosis drug, however, there are relatively few available data on PZA resistant (PZA-R) rate in China. From June 2009 to June 2012, we selected 493 isolates from five field settings in China to investigate PZA-R by pncA gene sequencing. The result showed that PZA-R rate was 1.0% (2/196) among pan-susceptible isolates, 3.1% (4/130) among isoniazid (INH) mono-resistant isolates, 14.0% (6/43) among rifampin (RIF) mono-resistant isolates and 43.5% (54/124) among multidrug resistant (MDR) isolates. MDR tuberculosis (TB), RIF mono-resistance, and retreatment were found to be risk factors for PZA-R. Newly diagnosed PZA-R TB patients and clustered isolates with identical pncA mutations indicate that transmission of PZA-R isolates plays an important role in emergence of PZA-R TB. The results suggest that, it is necessary to conduct PZA susceptibility test among MDR isolates and modify the treatment regimens accordingly.