By Richard R. Watkins, MD, MS, FACP, FIDSA
Professor of Internal Medicine, Northeast Ohio Medical University; Division of Infectious Diseases, Cleveland Clinic Akron General, Akron, OH
Dr. Watkins reports no financial relationships relevant to this field of study.
SYNOPSIS: In an experimental study, investigators found cigarette smoke increases the virulence of Staphylococcus aureus strains through several mechanisms, including augmented biofilm formation, increased invasion ability, and persistence within bronchial alveolar cells.
SOURCE: Lacoma A, Edwards AM, Young BC, et al. Cigarette smoke exposure redirects Staphylococcus aureus to a virulence profile associated with persistent infection. Sci Rep 2019;9:10798.
Cigarette smoking has a multitude of detrimental effects on human health, including an increased risk for certain infections such as pneumonia. The mechanisms underlying the increased risk are not fully elucidated, but they are thought to involve dysregulation of host immunity and damage to the integrity of the respiratory epithelium. Previous epidemiological studies have suggested an association between cigarette smoking and Staphylococcus aureus prevalence. Therefore, Lacoma and colleagues sought to assess the effects of cigarette smoke (CS) on specific virulence mechanisms important in the pathogenesis of S. aureus.
The study involved several in vitro experiments that used clinically significant lineages of methicillin-susceptible S. aureus (MSSA) and methicillin-resistant S. aureus (MRSA). CS was extracted and combined with tryptic soy broth (CS-TSB), which then was used to inoculate MRSA and MSSA cultures. The researchers then assessed biofilm formation, cytotoxicity, cell invasion and persistence, mutational frequency, and whole genome sequencing.
The investigators discovered that community-associated MRSA strains survived better within CS-TSB than hospital-associated MRSA strains. Inactivation of the accessory gene regulator (Agr), which is a global virulence regulator, had no effect on growth in CS-TSB. CS promoted biofilm formation, which was associated with decreased Agr activity. Growth in CS-TSB led to significantly decreased toxin production that corresponded to CS-mediated downregulation of Agr, although the extent of toxin decrease varied among the S. aureus strains. Furthermore, four strains grown in CS-TSB, but not regular TSB, demonstrated an increased invasive capacity in bronchial epithelial cells. Growth in CS-TSB also increased the population frequency of gentamicin-resistant small colony variants (SCVs) and rifampicin-resistant SCVs. Additional investigation of this mechanism determined that CS exposure increased the number of mutations in all of the tested strains, and reactive oxygen species within CS were major triggers for gentamicin-resistant SCVs. Interestingly, purified nicotine had no significant effect on SCV emergence. CS also affected cell wall teichoic acids (TAs), which play an important role in maintaining the cell wall of S. aureus. A TA mutant was highly sensitive to CS-TSB, suggesting that TAs mediate resistance to CS inhibition. Finally, when the growth media used for the SCVs was changed back to regular TSB, the majority of the colonies rapidly reverted back to wild-type.
This study elegantly describes the myriad changes that CS induces in S. aureus. The investigators found both the degree of CS exposure and the genetic background of the S. aureus strains are important in determining the effects of CS. This likely explains why some previous studies found CS to be a risk factor for S. aureus colonization, whereas others did not. For example, differences in the expression of teichoic acids in MRSA might be an important prerequisite for MRSA colonization in smokers. Furthermore, the observed resistance to gentamicin and rifampicin in the presence of CS also might affect other antibiotics, such as fluoroquinolones, for which resistance occurs through target site mutations.
In chronic rhinosinusitis, cigarette smoking has been identified as a significant risk factor, along with S. aureus SCVs. The findings from the present study provide a plausible explanation for recalcitrant disease, whereby CS induces the formation of SCVs leading to S. aureus colonization and resistance to antibiotic therapy. Notably, recent data have shown that CS induces double-stranded DNA damage in human cells in vivo. CS likely also causes similar damage to S. aureus DNA, leading to mutations and SCV emergence. Further in vitro and in vivo studies are warranted to test this hypothesis.
The study has a few limitations. For one, brief exposure to CS in vitro is unlikely to be equivalent to inhaled smoke over prolonged time periods. Another is that many other factors are involved with S. aureus colonization, which makes determining the role of confounding variables a challenge. It also would have been interesting to know if CS influenced the development of SCVs that were resistant to common drugs used for MRSA, such as doxycycline and trimethoprim-sulfamethoxazole.
Lacoma and colleagues provide valuable evidence of the deleterious effects of CS on S. aureus, which can be useful to clinicians when they are counseling patients to stop smoking. Therefore, smoking can be viewed as a modifiable risk factor that is especially important for patients with recurrent S. aureus infections, such as abscesses or rhinosinusitis. Smokers should be reminded that the best time to quit smoking was the day they started; the second best time to quit is today.