Loratadine Combats Methicillin-Resistant Staphylococcus aureus by Modulating Virulence, Antibiotic Resistance, and Biofilm Genes.

Methicillin-resistant Staphylococcus aureus (MRSA) has evolved to become resistant to multiple classes of antibiotics. New antibiotics are costly to develop and deploy, and they have a limited effective lifespan. Antibiotic adjuvants are molecules that potentiate existing antibiotics through nontoxic mechanisms. We previously reported that loratadine, the active ingredient in Claritin, potentiates multiple cell-wall active antibiotics in vitro and disrupts biofilm formation through a hypothesized inhibition of the master regulatory kinase Stk1. Loratadine and oxacillin combined repressed the expression of key antibiotic resistance genes in the bla and mec operons. We hypothesized that additional genes involved in antibiotic resistance, biofilm formation, and other cellular pathways would be modulated when looking transcriptome-wide. To test this, we used RNA-seq to quantify transcript levels and found significant effects in gene expression, including genes controlling virulence, antibiotic resistance, metabolism, transcription (core RNA polymerase subunits and sigma factors), and translation (a plethora of genes encoding ribosomal proteins and elongation factor Tu). We further demonstrated the impacts of these transcriptional effects by investigating loratadine treatment on intracellular ATP levels, persister formation, and biofilm formation and morphology. Loratadine minimized biofilm formation in vitro and enhanced the survival of infected Caenorhabditis elegans. These pleiotropic effects and their demonstrated outcomes on MRSA virulence and survival phenotypes position loratadine as an attractive anti-infective against MRSA.

Brominated Carbazole with Antibiotic Adjuvant Activity Displays Pleiotropic Effects in MRSA's Transcriptome.

Methicillin-resistant Staphylococcus aureus (MRSA) is a major threat to human health, as the US mortality rate outweighs those from HIV, tuberculosis, and viral hepatitis combined. In the wake of the COVID-19 pandemic, antibiotic-resistant bacterial infections acquired during hospital stays have increased. Antibiotic adjuvants are a key strategy to combat these bacteria. We have evaluated several small molecule antibiotic adjuvants that have strong potentiation with ß-lactam antibiotics and are likely inhibiting a master regulatory kinase, Stk1. Here, we investigated how the lead adjuvant (compound 8) exerts its effects in a more comprehensive manner. We hypothesized that the expression levels of key resistance genes would decrease once cotreated with oxacillin and the adjuvant. Furthermore, bioinformatic analyses would reveal biochemical pathways enriched in differentially expressed genes. RNA-seq analysis showed 176 and 233 genes significantly up- and downregulated, respectively, in response to cotreatment. Gene ontology categories and biochemical pathways that were significantly enriched with downregulated genes involved carbohydrate utilization, such as the citrate cycle and the phosphotransferase system. One of the most populated pathways was S. aureus infection. Results from an interaction network constructed with affected gene products supported the hypothesis that Stk1 is a target of compound 8. This study revealed a dramatic impact of our lead adjuvant on the transcriptome that is consistent with a pleiotropic effect due to Stk1 inhibition. These results point to this antibiotic adjuvant having potential broad therapeutic use in combatting MRSA.

Identification and Evaluation of Brominated Carbazoles as a Novel Antibiotic Adjuvant Scaffold in MRSA.

Antibiotic-resistant infections are a pressing global concern, causing millions of deaths each year. Methicillin-resistant Staphylococcus aureus (MRSA) is a leading cause of nosocomial infections in healthcare settings and is increasingly responsible for community-acquired infections that are often more difficult to treat. Antibiotic adjuvants are small molecules that potentiate antibiotics through nontoxic mechanisms and show excellent promise as novel therapeutics. Screening of low-molecular-weight compounds was employed to identify novel antibiotic adjuvant scaffolds for further elaboration. Brominated carbazoles emerged from this screening as lead compounds for further evaluation. Lead carbazoles were able to potentiate several ß-lactam antibiotics in three medically relevant strains of MRSA. Gene expression studies determined that these carbazoles were dampening the transcription of key genes that modulate ß-lactam resistance in MRSA. The lead brominated carbazoles represent novel scaffolds for elaboration as antibiotic adjuvants.