Christopher T. D. Price

The effects of salicylate on bacteria

Salicylate and related compounds, such as aspirin, have a variety of effects in eucaryotic systems and are well known for their medicinal properties. Salicylate also has numerous effects on bacteria, yet only a handful of individuals within the scientific community appreciate these findings. From a bacterial viewpoint, growth in the presence of salicylate can be both beneficial and detrimental. On one hand, growth of certain bacteria in the presence of salicylate can induce an intrinsic multiple antibiotic resistance phenotype. On the other hand, growth in the presence of salicylate can reduce the resistance to some antibiotics and affect virulence factor production in some bacteria. This review provides an overview of the effects salicylate has on various bacterial species.

Response of Staphylococcus aureus to Salicylate Challenge

Growth of Staphylococcus aureus with the nonsteroidal anti-inflammatory salicylate reduces susceptibility of the organism to multiple antimicrobials. Transcriptome analysis revealed that growth of S. aureus with salicylate leads to the induction of genes involved with gluconate and formate metabolism and represses genes required for gluconeogenesis and glycolysis. In addition, salicylate induction upregulates two antibiotic target genes and downregulates a multidrug efflux pump gene repressor (mgrA) and sarR, which represses a gene (sarA) important for intrinsic antimicrobial resistance. We hypothesize that these salicylate-induced alterations jointly represent a unique mechanism that allows S. aureus to resist antimicrobial stress and toxicity.

Pine oil cleaner-resistant Staphylococcus aureus: reduced susceptibility to vancomycin and oxacillin and involvement of SigB.

ABSTRACT Mutants of Staphylococcus aureus strain COL resistant to a household pine oil cleaner (POC) were isolated on laboratory media containing POC. S. aureus mutants expressing the POC resistance (POCr) phenotype also demonstrate reduced susceptibility to the cell wall-active antibiotics vancomycin and oxacillin. The POCr phenotype is reliant on the S. aureus alternative transcription factor SigB, since inactivation of sigB abolished expression of elevated POC resistance and the reductions in vancomycin and oxacillin susceptibilities. The isolation of suppressor mutants of COLsigB::kan, which maintain the sigB::kan allele, indicates that the POCr phenotype can also be expressed to a lesser degree via a sigB-independent mechanism. These results bolster a growing body of reports suggesting that common disinfectants can select for bacteria with reduced susceptibilities to antibiotics. A series of in vitro-selected glycopeptide-intermediate S. aureus (GISA) isolates also expressed reductions in POC susceptibility compared to parent strains. Viewed collectively, our evidence suggests that mutations leading to the POCr phenotype may also be involved with the mechanism that leads to the GISA phenotype.

Effects of salicylate and related compounds on fusidic acid MICs in Staphylococcus aureus

Salicylate, acetyl-salicylate, benzoate and ibuprofen increased fusidic acid MICs for fusidic acid-resistant and -susceptible strains of Staphylococcus aureus representing six genetic lineages. The effects of these substances on fusidic acid resistance levels occurred in a strain-dependent manner. The weak acid acetate, and acetaminophen did not alter fusidic acid resistance levels, while the addition of saligenin, the alcohol of salicylate, reduced gradient plate MICs for all strains studied. These findings indicate that a benzoic acid structure is required for the induction of increased intrinsic fusidic acid resistance levels. When 2 mM salicylate was added to media used in population analyses, the number of cells able to survive on high concentrations of fusidic acid increased. This increase in cell survival was observed in two unrelated fusidic acid-resistant strains, with chromosomal (WBG8287) or plasmid (WBG1576) mediated resistance determinants and two unrelated susceptible strains. The salicylate-induced increase in fusidic acid resistance was phenotypic at low fusidic acid concentrations (relative to resistance phenotype) for WBG8287 and a fusidic acid-susceptible strain. On media containing salicylate and high fusidic acid concentrations, the mutation frequency to higher fusidic acid resistance levels was greater for WBG8287, compared with unsupplemented fusidic acid-containing media. These experiments provide evidence for a novel salicylate inducible fusidic acid resistance mechanism in S. aureus.

Glycine Betaine Uptake by the ProXVWZ ABC Transporter Contributes to the Ability of Mycobacterium tuberculosis To Initiate Growth in Human Macrophages

Mycobacterium tuberculosis maintains a large genetic capacity necessary for growth in different environments during infection and survival upon aerosol transmission to new hosts. Screening for bacterial RNAs produced in response to host interactions produced candidate lists where we noted proXVWZ, annotated as encoding a putative glycine betaine or proline transporter. As high surface-to-volume ratios make bacterial cells particularly vulnerable to changes in water availability, we investigated the contributions of this transporter to the ability of M. tuberculosis to colonize macrophages. An H37Rv proXVWZ mutant was impaired for initial survival and intracellular growth and exhibited reduced growth at elevated medium osmolarity. This defect could be complemented by restoring proXVWZ and was attributable to a failure to accumulate the compatible solute glycine betaine. We then demonstrated that ProXVWZ allows M. tuberculosis to obtain betaine from host macrophages and thereby contributes to early steps in colonizing this niche.

Effects of opsonization of Rhodococcus equi on bacterial viability and phagocyte activation

OBJECTIVE To investigate the effect of opsonization of Rhodococcus equi with R. equi-specific antibodies in plasma on bacterial viability and phagocyte activation in a cell culture model of infection. SAMPLE Neutrophils and monocyte-derived macrophages from 6 healthy 1-week-old foals and 1 adult horse. PROCEDURES Foal and adult horse phagocytes were incubated with either opsonized or nonopsonized bacteria. Opsonization was achieved by use of plasma containing high or low concentrations of R. equi-specific antibodies. Phagocyte oxidative burst activity was measured by use of flow cytometry, and macrophage tumor necrosis factor (TNF)-α production was measured via an ELISA. Extracellular and intracellular bacterial viability was measured with a novel R. equi-luciferase construct that used a luminometer. RESULTS Opsonized bacteria increased oxidative burst activity in adult horse phagocytes, and neutrophil activity was dependent on the concentration of specific antibody. Secretion of TNF-α was higher in macrophages infected with opsonized bacteria. Opsonization had no significant effect on bacterial viability in macrophages; however, extracellular bacterial viability was decreased in broth containing plasma with R. equi-specific antibodies, compared with viability in broth alone. CONCLUSIONS AND CLINICAL RELEVANCE The use of plasma enriched with specific antibodies for the opsonization of R. equi increased the activation of phagocytes and decreased bacterial viability in the extracellular space. Although opsonized R. equi increased TNF-α secretion and oxidative burst in macrophages, additional factors may be necessary for effective intracellular bacterial killing. These data have suggested a possible role of plasma antibody in protection of foals from R. equi pneumonia.

Effect of blood on susceptibility to Staphylococcal endophthalmitis.

Purpose: The authors examined the effect of blood on susceptibility to experimental endophthalmitis. Methods: Forty rabbits received an injection of 5–25 colony-forming units of Staphylococcus epidermidis into the vitreous of the right eye. Twenty of these same eyes received a subsequent intravitreal injection of 0.2 mL blood while the remaining 20 received an intravitreal injection of 0.2 mL of a salt solution. All eyes were examined daily for signs of endophthalmitis. Vitreous cultures were obtained on day 2 from 30 of the 40 rabbits. Twenty rabbits were assigned for culture and euthanasia at day 5 and those remaining were cultured and killed at day 7. Results: In rabbits with blood and bacteria, 10 of 15 (67%) were culture positive at 2 days, compared to 2 of 15 (13%) that received salt solution and bacteria (P < 0.01). At days 5 and 7 there was no statistically significant difference in culture results. However, inflammatory scores were significantly higher at days 3–7 in rabbits with blood compared to those with salt solution (P ≤ 0.02). Conclusions: In this model, eyes with intravitreal blood were more likely to develop culture-positive endophthalmitis. Cultures were more likely to be positive at day 2, whereas inflammatory signs were more prevalent at days 5 and 7.