Anne Stone; Lisa Saiman

Curr Opin Pulm Med.  2007;13(6):515-521.  ?2007 Lippincott Williams & Wilkins

Abstract and Introduction

Abstract

Purpose of Review: Staphylococcus aureus is one of the first and most common pathogens to be isolated from the respiratory tract of patients with cystic fibrosis. The prevalence of respiratory tract colonization/infection with both methicillin-susceptible and methicillin-resistant S. aureus has increased over the past decade. The clinical significance of colonization/infection with these pathogens is variable, leading to numerous therapeutic strategies: primary prophylaxis, eradication, treatment of cystic fiboris pulmonary exacerbations, and treatment of methicillin-resistant S. aureus.
Recent Findings: Studies have demonstrated increased prevalence of S. aureus in clinical laboratories that use selective media. Additionally, small colony variant S. aureus has been associated with persistent infection, co-infection with Pseudomonas aeruginosa, and frequent courses of antibiotics, but this phenotype may be difficult to identify in clinical laboratories. Increased prevalence of methicillin-resistant S. aureus has led to use of oral and inhaled antibiotics in attempts to eradicate this pathogen; these studies have yielded variable results.
Summary: The epidemiology of S. aureus in cystic fibrosis has changed. Studies are needed to assess the clinical significance of the increased prevalence of both methicillin-susceptible and methicillin-resistant S. aureus, and whether primary prophylaxis or new treatment/eradication protocols are effective.

Introduction

Staphylococcus aureus is one of the most commonly isolated pathogens from the respiratory tract of patients with cystic fibrosis and one of the first microbes to infect the lungs of patients with cystic fibrosis.^[1] This review focuses on the role played by S. aureus in cystic fibrosis lung disease; recent research and current controversies surrounding the detection, treatment, and prevention of S. aureus infection, including infection with methicillin-resistant strains, are reviewed.

Epidemiology

In the pre-antibiotic era, infection of the airways with S. aureus was associated with significant morbidity and mortality in young children. With the advent of effective antistaphylococcal agents, routine use of antibiotics and other therapeutic interventions have improved the quality of life and prolonged the life expectancy of patients with cystic fibrosis.^[1] S. aureus continues to be one of the most commonly isolated pathogens from patients with cystic fibrosis, however. In 2005, S. aureus was isolated from respiratory tract secretions in 51.8% of all cystic fibrosis patients reported to the US Cystic Fibrosis Foundation Patient Registry; the prevalence of S. aureus colonization was highest among children and adolescent persons aged 17 years or younger.^[2] Moreover, the prevalence of S. aureus has increased during the past decade, as indicated in Fig. 1, as has the prevalence of methicillin-resistant S. aureus (MRSA). Among patients with cystic fibrosis reported to the Patient Registry, in 2001 MRSA was found to grow from respiratory tract secretions of only 7% of patients, compared with 17.2% of patients in 2005.^[1,2]

Figure 1. 

Prevalence of S. Aureus Isolated From the Respiratory Tracts of Patients With Cystic Fibrosis: 1995-2005

     

Ren et al.^[3*] analyzed data collected in 2001 from the Epidemiologic Study of Cystic Fibrosis, a large observational study of cystic fibrosis patients in the USA, and found that S. aureus was isolated as the sole pathogen in 7.5% of study patients. Of patients who were younger than 18 years, 90% of staphylococcal strains were methicillin-susceptible S. aureus (MSSA) and 10% were MRSA; among patients aged 18 years and older, corresponding percentages were 84% and 16%, respectively. In a single center study, Elizur et al.^[4*] reported that MRSA prevalence had increased from 22% in 2001 to 27% in 2004. Incidence rates of MRSA acquisition at this center were 4.5% in 2001, 6% in 2002, 4.3% in 2003, and 10.3% in 2004. Miall et al.^[5] reported an increased prevalence of MRSA from no patients in 1992 to seven patients in 1998. Thus, there is variability in the prevalence of MRSA between cystic fibrosis centers.

Risk Factors for Staphylococcus Aureus

Colonization of the anterior nares with S. aureus represents an important risk factor for subsequent infection in several patient populations, including infants hospitalized in the neonatal intensive care unit, patients undergoing cardiothoracic surgery, users of illicit intravenous drugs, and residents of chronic care facilities.^[6-10] In the general population, Kuehnert et al.^[11] reported a 36.9% prevalence of MSSA among children aged 1-19 years, with the peak prevalence occurring in children aged 6-11 years. MRSA carriage was only 0.8%.

Relatively few studies have investigated colonization of the anterior nares with S. aureus among people with cystic fibrosis. Goerke et al.^[12] examined 128 family members of 38 children with cystic fibrosis and 79 members of 23 non-cystic fibrosis families. The anterior nares of study participants were cultured four times over 19 months. In addition, 72 cystic fibrosis patients aged 1-25 years and 72 age-matched individuals without cystic fibrosis were cultured once. The investigators reported a significantly increased prevalence of nasal carriage of S. aureus among patients with cystic fibrosis who had not received antistaphylococcal antibiotics during the 4 weeks before culturing (66%), as compared with cystic fibrosis patients who had recently been treated (29%) and healthy individuals without cystic fibrosis (32%; P < 0.001). In addition, the study reported that 73% of 28 individuals with S. aureus in their sputum had S. aureus in their nares; sputum and nares isolates were the same clone in 86%. In 55% of cystic fibrosis families and 62% of non-cystic fibrosis families, two or more family members shared the same S. aureus clone, as assessed using pulse field gel electrophoresis. This suggests that family members are a source of acquisition of S. aureus, and the nose may be a source for subsequent lower airway colonization/infection.

For decades MRSA has caused infections in non-cystic fibrosis patients with traditional risk factors, such as hospitalization, surgery, residence in a chronic care facility, or intravenous drug use.^[6,10] Over the past several years, MRSA infections have been reported in otherwise healthy patients, including children and young adults, who lack traditional risk factors^[13-16] These so-called community-acquired MRSA strains have a distinct genotype and phenotype relative to hospital-acquired MRSA. Unlike multidrug-resistant hospital-acquired MRSA, in which treatment options are limited, community-acquired MRSA strains are generally susceptible to numerous antimicrobial agents, including clindamycin, fluoroquinolones, and trimethoprim-sulfamethoxazole (co-trimoxazole)^[14,15] It is unknown whether the increased MRSA prevalence among cystic fibrosis patients results from infection with community-acquired or hospital-acquired strains. This distinction is important because it could affect future therapeutic and preventive strategies.

Microbiology

Accurate identification of pathogens from respiratory tract secretions of patients with cystic fibrosis is essential in establishing accurate epidemiologic data and guiding therapeutic interventions and infection control. In 2001 the Cystic Fibrosis Foundation updated previous recommendations for clinical microbiology, including the use of selective media for staphylococci to optimize isolation of these pathogens, because mucoid Pseudomonas aeruginosa may obscure the growth of slower growing pathogens^[17,18**] The Cystic Fibrosis Foundation recommended mannitol salt agar, which uses sodium chloride as a selective agent, and phenol red as an indicator of mannitol utilization to distinguish S. aureus from nonpathogenic staphylococci. Columbia/colistin-nalidixic acid media is also selective for staphylococci, but it lacks an indicator reagent. The guidelines endorse the use of agar containing oxacillin to screen for MRSA^[17]

Zhou et al.^[19*] reviewed compliance of clinical microbiology laboratories in the USA with published recommendations for the processing of cystic fibrosis respiratory specimens. Microbiology laboratory protocols from 150 out of 190 cystic fibrosis centers were reviewed; 82% used selective media to identify S. aureus. This represents an increase compared with data collected in 1995 by the Epidemiologic Study of Cystic Fibrosis,^[20] which demonstrated that 65% of sites used selective media for sputum cultures and 51% of sites for throat swabs. The study also demonstrated that the prevalence of S. aureus was 54% at sites adhering to complete protocols (e.g. use of throat swabs from patients who did not produce sputum and use of selective media), but only 48% at sites using partial protocols. Similar findings were reported by Vergison et al.^[21**] in Belgium; the prevalence of S. aureus ranged from 20% to 70% among nine cystic fibrosis centers, and centers that used selective media identified greater rates of S. aureus prevalence.

Additionally, small colony variants (SCVs) of S. aureus have been isolated from patients with cystic fibrosis. Such strains have been associated with persistent infection and frequent courses of antibiotics, increased antibiotic resistance, and more advanced lung disease.^[21**,22**] Although their clinical significance is not fully understood, studies have demonstrated that SCV strains may be difficult to detect in the clinical laboratory because of their phenotypic characteristics (i.e. slow growth, failure to use mannitol, and tendency to form nonpigmented, nonhemolytic colonies with reduced coagulase production)^{[22**,23,24**,25]} Moreover, there is evidence that co-infection with P. aeruginosa within the cystic fibrosis lung selects for S. aureus SCVs and confers aminoglycoside resistance to S. aureus because of production of the exoproduct 4-hydroxy-2-heptylquinolone-N-oxide, which inhibits electron transport-mediated aminoglycoside uptake^[24**]

Virulence Factors

S. aureus expresses numerous virulence factors to evade host defenses and cause damage to host tissues, as described in Table 1 and reviewed by Foster.^[26] The relevant contributions of different S. aureus virulence factors in cystic fibrosis infections and changes in virulence factor expression in initial versus chronic infection are unknown. Of note, community-acquired MRSA strains frequently express the virulence factor Panton-Valentine leukocidin (PVL), which has been associated with severe skin infections and necrotizing pneumonia.^[13,27-29] PVL is a cytolytic toxin that forms pores in the membranes of leukocytes, leading to cell lysis.^[26] PVL-positive MRSA strains have been isolated from respiratory tract specimens of patients with cystic fibrosis.^[4*]

An intriguing observation has been reported by Prunier et al.^[30] when comparing strains of S. aureus isolated from patients with cystic fibrosis versus those from patients without cystic fibrosis (blood or respiratory tract cultures). Cystic fibrosis strains with resistance to macrolides were unusual in that they did not express common resistance mechanisms, such as an efflux pump or methylase gene, but rather they expressed mutations in the rRNA target. In addition, these investigators reported a greater frequency of resistance to rifampin (13/89 cystic fibrosis isolates vs. 1/74 non-cystic fibrosis isolates; P = 0.0045) and streptomycin (9/89 cystic fibrosis isolates vs. 1/74 non-cystic fibrosis isolates; P = 0.04) among cystic fibrosis strains. These investigators attributed the high rates of antibiotic resistance to hypermutable cystic fibrosis strains with mutations or deletions in mutS; mut genes improve the accuracy of DNA replication by initiating the repair of base pair mismatches, insertions, or deletions. Prunier et al. hypothesized that mutS mutations (or other undescribed genetic mechanisms that lead to the hypermutable phenotype) confer a selective advantage within the cystic fibrosis lung.^[30] Hypermutable strains of P. aeruginosa have also been described in cystic fibrosis.^[31]

Treatment of S. Aureus in Cystic Fibrosis

There are several potential indications for antibiotic treatment of S. aureus in cystic fibrosis. These include primary prophylaxis, eradication, treatment of cystic fibrosis pulmonary exacerbations, and treatment of the multidrug-resistant pathogen MRSA. To date, there are no descriptions of chronic suppressive therapy such as that used for P. aeruginosa infection. Antistaphylococcal antibiotics are summarized in Table 2 .

Primary Prophylaxis for S. Aureus in Cystic Fibrosis

Several studies have evaluated primary prophylaxis for S. aureus using a variety of agents, as reviewed by Smyth.^[32] Overall, these studies demonstrated a reduction in acquisition of S. aureus among children receiving antibiotics. Many studies were not randomized or of high statistical power.^[32,33] In addition, other beneficial clinical outcomes such as fewer hospitalizations or improved lung function were not consistently observed.^[32] The most serious concern with the chronic use of antistaphylococcal antibiotics is the potential increased risk for infection with P. aeruginosa. Stutman et al.^[34] reported increased P. aeruginosa acquisition in cystic fibrosis children followed from age 2 to 6 years treated with continuous cephalexin versus placebo. This effect was not observed with flucloxacillin prophylaxis.^[32,35] Ratjen et al.^[33] described an increased risk for P. aeruginosa acquisition, particularly in children younger than 6 years, using several antistaphylococcal agents; 42.6% of the 308 patients were treated with cephalosporins, 26.4% with co-trimoxazole, 14.6% with macrolides, 8.5% with amoxicillin/clavulanic acid, 3.3% with flucloxacillin, and 4.6% with other agents. Differences in the study findings may reflect the duration of treatment and the type of agent used; cephalexin was used for a longer period of time and is more broad-spectrum than flucloxacillin. Currently, the UK Cystic Fibrosis Trust Antibiotic Group recommends the use of flucloxacillin from the time of diagnosis of cystic fibrosis until 2 years of age to reduce cough, use of other antibiotics, and acquisition of S. aureus infection.^[32,36] Antistaphylococcal prophylaxis is not routinely practiced in North America.

Eradication

Several studies have demonstrated eradication of S. aureus from respiratory secretions with the use of antistaphyloccocal antibiotics, but they failed to demonstrate a concomitant improvement in pulmonary function or other clinical outcomes.^[37-40] The UK Cystic Fibrosis Trust Antibiotic Group^[36] recommended a 2-week course of antibiotics to eradicate S. aureus as an alternative to prophylactic therapy. As evidence, these guidelines cite the low rate of chronic S. aureus infection in Copenhagen, where patients undergo monthly surveillance cultures of their respiratory tract secretions and S. aureus is treated promptly when detected. Eradication of S. aureus is not routinely performed in North America.

Pulmonary Exacerbations

Antibiotics targeted against S. aureus are commonly chosen for the treatment of pulmonary exacerbations both in patients with cystic fibrosis who are infected with this pathogen^[41] and empirically in young children because of the high prevalence of this pathogen. Children often are not sputum producers; therefore, oropharyngeal swabs are frequently obtained. Studies have compared oropharyngeal swabs with specimens obtained by bronchoalveolar lavage; cultures obtained from the upper airway correlated with lower airway infection.^[42-44] Ramsey et al.^[42] reported a 91% positive predictive value [95% confidence interval (CI) 59-100] for S. aureus identified by oropharyngeal swab compared with bronchial cultures and an 80% negative predictive value (95% CI 52-96). This suggests that oropharyngeal swabs with positive cultures are highly predictive, but negative cultures do not rule out the presence of a pathogen. Studies have not compared the genotypes of S. aureus isolated from bronchoalveolar lavage with those from oropharyngeal cultures.^[44,45]

Methicillin-resistant S. Aureus Eradication and Therapy

Although the role played by MRSA in the progression of lung disease in cystic fibrosis is not clearly understood, some studies have suggested that MRSA colonization/infection is associated with a worsened clinical course^[3*,4*,5] Miall et al.^[5] compared 10 children with cystic fibrosis and MRSA vs. 18 patients with cystic fibrosis negative for MRSA; they found deteriorations in height, weight, and BMI in the MRSA positive group, but only the change in height was statistically significant (P = 0.039). The children with MRSA received more courses of intravenous antibiotics in the year after MRSA acquisition compared with control children (P = 0.046). Ren et al.^[3*] described increased hospitalization and antibiotic use in addition to lower mean predicted fractional expiratory volume in 1 s (P < 0.001) in both children and adults with cystic fibrosis who harbored MRSA compared with those harboring MSSA. Additionally, Elizur et al. reported that six out of 40 new isolates of MRSA in cystic fibrosis patients expressed the virulence factor PVL.^[4*] Patients with newly acquired PVL-positive MRSA were more likely to be hospitalized for a pulmonary exacerbation (P < 0.01), had more focal infiltrates on chest radiograph (P = 0.04), and exhibited a greater decline in lung function (P = 0.01) compared with cystic fibrosis patients with PVL-negative MRSA strains.^[4*] Thus, there has been interest in the development of more effective eradication and treatment strategies for MRSA.

Case reports and clinical studies have reported successful eradication or suppressive therapy aimed at treating MRSA.^[46-49] Most clinical studies, however, have involved small numbers of cystic fibrosis patients with variable follow up. Decolonization following treatment with rifampicin and fusidic acid for 6 months has been documented in an observational study of seven adults with cystic fibrosis; five of the seven patients were culture negative during treatment and as long as 6 months after treatment.^[46] This study also reported a decreased number of days of parenteral antibiotics during and after treatment when compared with usage before the decolonization protocol. The roles of both oral and nebulized vancomycin have also been explored.^[47,48] Nebulized vancomycin in combination with oral therapy and long-term prophylaxis was investigated in a retrospective 12-year study of 15 children with cystic fibrosis; 12 out of 15 children received cephadine and 5 days of oral and nebulized vancomycin, associated with a 55% eradication rate.^[48] Reports of S. aureus isolates with intermediate susceptibility to vancomycin, so-called 'VISA' strains, and the increasing prevalence of vancomycin-resistant enterococci serve as a caution to use this agent judiciously.^[50]

Eradication of MRSA with oral linezolid has been reported in adult patients with cystic fibrosis.^[51] Twice daily linezolid compared with vancomycin for the treatment of MRSA infections in patients without cystic fibrosis was associated with shorter hospitalizations and fewer days of intravenous therapy.^[52] A study conducted in adults with cystic fibrosis demonstrated adequate serum and sputum concentrations with twice daily dosing.^[53] In another study of adults with cystic fibrosis, Bosso et al.^[54] demonstrated that the half-life of linezolid varied from less than 2 h to more than 8 h after a single intravenous dose of 600 mg; similar findings were noted in other patient populations. The use of linezolid among cystic fibrosis patients with MRSA needs further investigation, particularly with regard to safety, but it offers the potential benefit of an oral agent without the toxicities and need for drug monitoring associated with vancomycin. The emergence of MRSA resistant to linezolid has been reported in a child with cystic fibrosis treated for 55 days following treatment with multiple courses of this agent over 18 months.^[55*]

Impact of Aerosolized Antibiotic and Anti-inflammatory Therapies on S. Aureus

The impact of chronic suppressive therapies such as azithromycin and inhaled tobramycin on S. aureus infection in cystic fibrosis is unclear. In patients colonized with P. aeruginosa, azithromycin has been found to improve lung function, reduce pulmonary exacerbations, and reduce antibiotic use.^[56] In the randomized, placebo-controlled US trial of azithromycin,^[57] 10% fewer participants in the azithromycin group had treatment-emergent S. aureus when compared with participants in the placebo group. However, there was no difference in S. aureus eradication. Additionally, the emergence or eradication of MRSA did not differ between treatment and placebo groups. Recent reports, however, have described the emergence of macrolide resistance among S. aureus strains isolated from patients with cystic fibrosis treated with chronic azithromycin.^[35,58] Prunier et al.^[58] examined 24 S. aureus strains isolated from respiratory tract specimens from nine patients with cystic fibrosis who had received azithromycin; they noted that 20 (83%) were resistant to erythromycin. Notably, only three patients had a susceptible strain documented before azithromycin use. Clement et al.^[59*] investigated the use of azithromycin in cystic fibrosis patients with and without P. aeruginosa infection. They reported a decrease in pulmonary exacerbations, as defined as a decrease in the use of oral antibiotics, regardless of P. aeruginosa status (P < 0.01); there were no significant differences in the proportion of patients in the active versus placebo group with S. aureus at 6 and 12 months.

Gibson et al.^[60] recently investigated the safety and efficacy of aerosolized tobramycin for early eradication of P. aeruginosa. Of the eight patients in the treatment group, five were co-infected with S. aureus before therapy; three out of the five patients had negative cultures for S. aureus on day 28 of treatment. This pilot study did not investigate long-term clinical outcomes or suggest that chronic inhaled tobramycin was effective against S. aureus.

Ramsey et al.^[61] did not report the impact of chronic aerosolized tobramycin on S. aureus among 520 adult patients with cystic fibrosis infected with P. aeruginosa. Similarly, Burns et al.^[62] did not examine S. aureus emergence or eradication in their analysis of the microbiology results of these data, and neither did Ramsey et al.^[63] in a smaller crossover study of 71 adult cystic fibrosis patients treated with aerosolized tobramycin.

Thus, the effect on S. aureus of early eradication strategies or chronic suppressive therapies directed against P. aeruginosa remains unclear and warrants further investigation.

Prevention

There has been interest in the development of a vaccine to protect against S. aureus infection in the non-cystic fibrosis community, specifically because S. aureus virulence factors prevent a strong antibody response and immunologic memory.^[26] In animal models, active immunization has been shown to lead to production of antibodies after stimulation with S. aureus surface components.^[26] Passive immunization with human immunoglobulin also has been demonstrated to be effective in mice, and clinical trials are ongoing in low birth weight neonates.^[26,64-66] Moreover, a humanized monoclonal antibody targeted against the S. aureus surface polysaccharides type 5 and 8 and conjugated to a carrier protein has been shown to protect hemodialysis patients from S. aureus bacteremia for as long as 10 months.^[26,67] To date, no published studies of vaccination against S. aureus have been performed in cystic fibrosis patients.

Infection control strategies to prevent MRSA transmission among cystic fibrosis patients or between cystic fibrosis and non-cystic fibrosis patients were recently reviewed.^[1,17]

Conclusion

S. aureus is commonly isolated from the respiratory tract of cystic fibrosis patients and is one of the first pathogens to colonize/infect the cystic fibrosis lung. Over recent years, S. aureus has been isolated from the cystic fibrosis lung with increasing frequency, potentially reflecting the enhanced compliance of clinical microbiology laboratories with standardized processing. MRSA strains have become more prevalent, and although some studies have suggested that MRSA is associated with increased airway obstruction, increased antibiotic use, and worsened clinical courses, the impact of MRSA remains unclear. Further studies are needed to determine the optimal management of MSSA and MRSA in patients with cystic fibrosis.