Molecular Mechanisms of Staphylococcus and Pseudomonas Interactions in Cystic Fibrosis

Abstract

Cystic fibrosis (CF) is an autosomal recessive genetic disorder that is characterized by recurrent and chronic infections of the lung predominantly by the opportunistic pathogens, Gram-positive Staphylococcus aureus and Gram-negative Pseudomonas aeruginosa. While S. aureus is the main colonizing bacteria of the CF lungs during infancy and early childhood, its incidence declines thereafter and infections by P. aeruginosa become more prominent with increasing age. The competitive and cooperative interactions exhibited by these two pathogens influence their survival, antibiotic susceptibility, persistence and, consequently the disease progression. For instance, P. aeruginosa secretes small respiratory inhibitors like hydrogen cyanide, pyocyanin and quinoline N-oxides that block the electron transport pathway and suppress the growth of S. aureus. However, S. aureus survives this respiratory attack by adapting to respiration-defective small colony variant (SCV) phenotype. SCVs cause persistent and recurrent infections and are also resistant to antibiotics, especially aminoglycosides, antifolate antibiotics, and to host antimicrobial peptides such as LL-37, human β-defensin (HBD) 2 and HBD3; and lactoferricin B. The interaction between P. aeruginosa and S. aureus is multifaceted. In mucoid P. aeruginosa strains, siderophores and rhamnolipids are downregulated thus enhancing the survival of S. aureus. Conversely, protein A from S. aureus inhibits P. aeruginosa biofilm formation while protecting both P. aeruginosa and S. aureus from phagocytosis by neutrophils. This review attempts to summarize the current understanding of the molecular mechanisms that drive the competitive and cooperative interactions between S. aureus and P. aeruginosa in the CF lungs that could influence the disease outcome.

Keywords: Pseudomonas aeruginosa; Staphylococcus aureus; adaptation; antagonism; co-existence; cystic fibrosis.

Figure 1

Prevalence of Respiratory Microorganisms by age cohort, 2020. The figure shows the prevalence of microorganisms in the respiratory tract of CF patients in different age groups. Even at a very young age, affected individuals have at least one microorganism, and this number increases with increasing age. S. aureus is by far the most widespread microorganism, which is followed with time by P. aeruginosa. What one can also see is that the incidence of MRSA is only about 20% of that of MSSA. Hemophilus influencae, Achromobacter, Burkholderia cepacia and Stenotrophomonas maltophilia were only rarely observed. This image is reproduced with permission from Cystic Fibrosis Foundation Patient Registry, 2020 Annual Data Report, Bethesda, Maryland^©2021 Cystic Fibrosis Foundation.

Figure 2

Effect of Long chain - acyl homo serine lactones (AHLs) of P. aeruginosa on S. aureus. In general, SigB and SarA regulate the expression of a number of genes and their products and enhance the virulence of S. aureus. AHLs of P. aeruginosa target SigB and SarA and cause decreased transcription of sigB, sarA and agr RNAIII resulting in decreased capsule production, biofilm formation and dissemination. Inhibition of agr RNAIII enhances the production of Protein A, ClfB, FnbpAB, which increase the adhesion of S. aureus to host cells. RpiRc regulates RsbU to modulate eDNA-dependent biofilm formation and in vivo virulence of S. aureus in a mouse model of catheter infection. The network was drawn based on the knowledge acquired from literature (Schmidt et al., 2003; Tamber and Cheung, 2009; Fechter et al., 2014; Andrey et al., 2015). Arrows represent activation while bars represent repression. SPs & M, Secreted proteins & metabolites; eDNA, extracellular DNA; ClfB, clumping factor B; FnbpAB, Fibronectin binding proteins A and B; spa, Protein A; hla, alpha-toxin; sae, S. aureus exoproteins.

Figure 3

Effect of respiratory inhibitors of P. aeruginosa on S. aureus. P. aeruginosa secretes small respiratory inhibitors such as pyocyanin (PYO), hydrogen cyanide (HCN) and quinoline N-oxides (HQNO) that target the electron transport pathway (ETC) and block the transport of electrons. S. aureus shifts to fermentative metabolism and adapts the small colony variant (SCV) phenotype to survive. In addition to resisting the respiratory inhibitors of P. aeruginosa, SCV phenotype imparts several advantages to S. aureus such as increased antibiotic resistance and persistence. The lactate that is produced by S. aureus SCVs will be consumed by P. aeruginosa as a preferential carbon source. PG, peptidoglycan; CM, cell memebrane.

Figure 4

Effect of staphylolysin (LasA) of P. aeruginosa on S. aureus. P. aeruginosa secreted staphylolysin (LasA) targets the glycyl-glycine and glycyl-alanine bonds of the pentaglycine cross-linkage in the peptidoglycan of S. aureus and induces its lysis. Lysed S. aureus cells could serve as a source of iron for P. aeruginosa thus promotes its growth. The N-acetyl glucosamine (GlcNAc) that is shed will be sensed by P. aeruginosa and enhances its PQS quorum sensing system and induces the production of extracellular virulence factors such as PYO, rhamnolipids and LasA etc that will aid in eliminating its competitors. S. aureus adapts the L-form like colonies to survive. In addition to surviving the effects of LasA, L-form like colonies confer several advantages to S. aureus such as evasion of phagocytosis, enhanced intracellular survival and protection from the effect of extracellular antibiotics.

Figure 5

Co-operative interactions between S. aureus and P. aeruginosa. (1) S. aureus induces mutations in the genes encoding the lipopolysaccharide (LPS) biosynthesis in P. aeruginosa that confers fitness gain and enhanced resistance to β-lactam antibiotics. (2) Protein A (SpA) produced by S. aureus interacts with the Psl (Pseudomonas polysaccharide locus) polysaccharide of P. aeruginosa and protects it from being phagocytosed by neutrophils. (3) SpA interacts with type IV pili of P. aeruginosa and inhibits biofilm formation by P. aeruginosa strains. (4) Exogenous alginate secreted by P. aeruginosa protects S. aureus by reducing the production of virulence factors such as siderophores, and rhamnolipids.