Staphylococcus aureus persisters are associated with reduced clearance in a catheter-associated biofilm infection

Background

Staphylococcus aureus causes a wide variety of infections, many of which are chronic or relapsing in nature. Antibiotic therapy is often ineffective against S. aureus biofilm-mediated infections. Biofilms are difficult to treat partly due to their tolerance to antibiotics, however the underlying mechanism responsible for this remains unknown. One possible explanation is the presence of persister cells-dormant-like cells that exhibit tolerance to antibiotics. Recent studies have shown a connection between a fumC (fumarase C, a gene in the tricarboxylic acid cycle) knockout strain and increased survival to antibiotics, antimicrobial peptides, and in a Drosophila melanogaster model.

Objective

It remained unclear whether a S. aureus high persister strain would have a survival advantage in the presence of innate and adaptive immunity. To further investigate this, a fumC knockout and wild type strains were examined in a murine catheter-associated biofilm model.

Results

Interestingly, mice struggled to clear both S. aureus wild type and the fumC knockout strains. We reasoned both biofilm-mediated infections predominantly consisted of persister cells. To determine the persister cell population within biofilms, expression of a persister cell marker (Pcap5A::dsRED) in a biofilm was examined. Cell sorting of biofilms challenged with antibiotics revealed cells with intermediate and high expression of cap5A had 5.9-and 4.5-fold higher percent survival compared to cells with low cap5A expression. Based on previous findings that persisters are associated with reduced membrane potential, flow cytometry analysis was used to examine the metabolic state of cells within a biofilm. We confirmed cells within biofilms had reduced membrane potential compared to both stationary phase cultures (2.5-fold) and exponential phase cultures (22.4-fold). Supporting these findings, cells within a biofilm still exhibited tolerance to antibiotic challenge following dispersal of the matrix through proteinase K.

Conclusion

Collectively, these data show that biofilms are largely comprised of persister cells, and this may explain why biofilm infections are often chronic and/or relapsing in clinical settings.

Interruption of the tricarboxylic acid cycle in Staphylococcus aureus leads to increased tolerance to innate immunity

Staphylococcus aureus is widely known for its resistance and virulence causing public health concerns. However, antibiotic tolerance is also a contributor to chronic and relapsing infections. Previously, it has been demonstrated that persister formation is dependent on reduced tricarboxylic acid (TCA) cycle activity. Persisters have been extensively examined in terms of antibiotic tolerance but tolerance to antimicrobial peptides (AMPs) remains largely unexplored. AMPs are a key component of both the human and Drosophila innate immune response. TCA cycle mutants were tested to determine both antibiotic and AMP tolerance. Challenging with multiple classes of antibiotics led to increased persister formation (100- to 1,000-fold). Similarly, TCA mutants exhibited AMP tolerance with a 100- to 1,000-fold increase in persister formation when challenged with LL-37 or human β-defensin 3 (hβD3). The ability of TCA cycle mutants to tolerate the innate immune system was further examined with a D. melanogaster model. Both males and females infected with TCA cycle mutants exhibited increased mortality and had higher bacterial burdens (1.5 log) during the course of the infection. These results suggest increasing the percentage of persister cells leads to increased tolerance to components of the innate immune system.

Polymicrobial Interactions Induce Multidrug Tolerance in Staphylococcus aureus Through Energy Depletion

Staphylococcus aureus is responsible for a high number of relapsing infections, which are often mediated by the protective nature of biofilms. Polymicrobial biofilms appear to be more tolerant to antibiotic treatment, however, the underlying mechanisms for this remain unclear. Polymicrobial biofilm and planktonic cultures formed by S. aureus and Candida albicans are 10- to 100-fold more tolerant to oxacillin, vancomycin, ciprofloxacin, delafloxacin, and rifampicin compared to monocultures of S. aureus. The possibility of C. albicans matrix components physically blocking antibiotic molecules from reaching S. aureus was ruled out as oxacillin, ciprofloxacin, delafloxacin, and rifampicin were able to diffuse through polymicrobial biofilms. Based on previous findings that S. aureus forms drug tolerant persister cells through ATP depletion, we examined nutrient deprivation by determining glucose availability, which indirectly correlates to ATP production via the tricarboxylic acid (TCA) cycle. Using an extracellular glucose assay, we confirmed that S. aureus and C. albicans polymicrobial cultures depleted available glucose faster than the respective monocultures. Supporting this finding, S. aureus exhibited decreased TCA cycle activity, specifically fumarase expression, when grown in the presence of C. albicans. In addition, S. aureus grown in polymicrobial cultures displayed 2.2-fold more cells with low membrane potential and a 13% reduction in intracellular ATP concentrations than in monocultures. Collectively, these data demonstrate that decreased metabolic activity through nutrient deprivation is a mechanism for increased antibiotic tolerance within polymicrobial cultures.