Autoinducer-2 (AI-2) is a small diffusible quorum-sensing molecule produced by several bacteria, including staphylococci, and has been shown to cause upregulation of several major virulence genes of P. aeruginosa discussed above, including extracellular protease (lasB), rhamnosyltransferase involved in rhamnolipid synthesis (rhlA), exotoxins (exoT, exoS, exoY) and phenazines (phzA1 and phzA2; Li et al., 2015). Induction of P. aeruginosa virulence by AI-2 was shown both in vitro after screening of a random lux reporter-based promotor library and in vivo in rat lung infection and Drosophila chronic infection models (Duan et al., 2003; Sibley et al., 2008; Li et al., 2015). AI-2 mediated quorum-sensing is now recognized as a universal language of interspecies communication regulating a wide variety of genes involved in virulence and biofilm formation in a cell-density dependent manner in a number of micro-organisms, including non-producers like P. aeruginosa (Rezzonico et al., 2012). Furthermore, AI-2 has been detected in substantial amounts in the sputum of CF patients and in infected rats (Duan et al., 2003), raising the possibility of interruption of AI-2 signaling to either slow down disease progression or hasten the healing process.

A promising approach is the use of AI-2 analogs like D-ribose that block the AI-2 pathway and inhibit P. aeruginosa virulence. Wang et al showed in a rat model of mechanical ventilation that co-inoculation of P. aeruginosa and Streptococcus mitis resulted in increased biomass, lung damage, and rat mortality compared to infection with only P. aeruginosa (Wang et al., 2016). Treatment with D-ribose of both the single and dual species infections showed a significant decrease in biomass and lowering of rat mortality in the latter group due to interference with AI-2 signaling (Wang et al., 2016). Inhibition of P. aeruginosa virulence is not only beneficial because the P. aeruginosa-mediated damage is reduced but also because the immune system is less stimulated (Figure 1). Further studies showing the benefits of non-toxic biofilm inhibitors such as D-ribose in patient populations are awaited.

Another molecule that increases the virulence of P. aeruginosa and is commonly found in the CF lung is N-acetyl glucosamine (GlcNAc). GlcNAc is part of the Gram-positive cell wall polymer peptidoglycan and induces the virulence of P. aeruginosa by enhancing the Pseudomonas quinolone signal (PQS), which controls the production of extracellular virulence factors like pyocyanin, elastase, rhamnolipids and HQNO (discussed in Section Pseudomonas Quinolone Signal Regulates the Production of Anti-Staphylococcal 4-Hydroxy-2-Heptylquinoline N-Oxide (HQNO); Deziel et al., 2004; Williams and Camara, 2009; Jimenez et al., 2012; Korgaonkar et al., 2013; Figures 1, 2). The PQS, with 2-heptyl-3-hydroxy-4-quilonone as the main effector molecule, is one of the three quorum-sensing systems present in P. aeruginosa. PQS is positively regulated by LasR and negatively regulated by RhlR, the two other quorum-sensing systems of P. aeruginosa with N-acylhomoserine lactone as main effector molecule (Jimenez et al., 2012). P. aeruginosa has the ability to sense the peptidoglycan shed by the Gram positive commensal flora and in response increase the production of antimicrobials. The enhanced virulence in the presence of GlcNAc from Gram-positive bacteria was demonstrated in vivo in a Drosophila and Galleria mellonella infection model (Korgaonkar et al., 2013; Whiley et al., 2014). Both GlcNAc and AI-2 sensing are examples where P. aeruginosa can sense its environment and generate the appropriate response to eliminate competitors by producing several virulence factors, which also has a negative impact on the host.

P. aeruginosa strongly reduces or completely outcompetes S. aureus during co-culture in many in vitro model systems, both planktonic and biofilm (Palmer et al., 2005; Baldan et al., 2014; DeLeon et al., 2014). This anti-staphylococcal activity of P. aeruginosa was first described by Lightbown and Jackson (1956), who identified 4-hydroxy-2-heptylquinoline N-oxide (HQNO) as a major compound produced by P. aeruginosa that inhibited the cytochrome systems of some bacteria, including S. aureus (Lightbown and Jackson, 1956). The same phenomenon was again described by Machan et al. in 1991 by testing the culture supernatant of fifty P. aeruginosa clinical isolates on 261 staphylococci (Machan et al., 1991). The growth of all staphylococci was reduced by each of the P. aeruginosa strains, although the extent of inhibition was strain-dependent. The factor responsible for this phenomenon was again identified as HQNO (Machan et al., 1992). HQNO is the major compound produced by the pqsABCDE operon, which is regulated by the quorum-sensing system PQS. Although HQNO is described as an antistaphylococcal compound, it has no lytic activity against S. aureus itself but rather slows down the growth by inhibiting oxidative respiration (Figure 2, right upper panel; Williams and Camara, 2009). Exposure to an HQNO source suppresses the growth of S. aureus, resulting in small colonies which are easily missed in diagnostic cultures (Hoffman et al., 2006). These so-called SCVs represent a different phenotype with specific characteristics and will be discussed later in this review. Furthermore, HQNO can be detected at active concentrations in the sputum of CF patients infected with P. aeruginosa, suggesting that HQNO has the same effect in the lungs of CF patients as it has in vitro (Hoffman et al., 2006). Although HQNO is one of the most important and well-studied antistaphylococcal compounds, it is not the only factor slowing down the growth and inhibiting oxidative respiration in staphylococci.

Pyocyanin is one of the numerous pigmented phenazines produced by P. aeruginosa and an important virulence factor. Pyocyanin is produced during Pseudomonas biofilm formation, has a role in acute and chronic airway infections, enables anaerobic survival and serves as a redox-active antimicrobial compound (Biswas et al., 2009; Caldwell et al., 2009). Furthermore, by its inter-and intracellular signaling, pyocyanin enables P. aeruginosa to successfully compete with other bacteria and even fungi (Gibson et al., 2009; Toyofuku et al., 2010; Tashiro et al., 2013). Like HQNO, pyocyanin also blocks the oxidative respiration and inhibits growth of S. aureus, also selecting for the SCV phenotype (Biswas et al., 2009; Figure 2, right upper panel). The production of pyocyanin can be observed after 8 h of culture, around the same time when a strong reduction of S. aureus cells occurs during co-culture (Biswas et al., 2009; Tashiro et al., 2013). Furthermore, the presence of Gram-positive organisms, including some Staphylococcus spp., can induce pyocyanin production in P. aeruginosa by stimulating the PQS system (Korgaonkar and Whiteley, 2011; Whiley et al., 2014; Figure 2, middle panel). In addition, exposure to pyocyanin in the airways leads to pulmonary damage and contributes to CF pathogenesis (Caldwell et al., 2009). Therefore, pyocyanin seems to be an antagonistic compound secreted to provide a competitive advantage to P. aeruginosa by harming S. aureus, other Gram-positive bacteria as well as the host.

P. aeruginosa secretes a staphylolytic endopeptidase called LasA protease or staphylolysin, which degrading pentaglycine in the cell wall of S. aureus causing cell lysis (Kessler et al., 1993). P. aeruginosa might use LasA protease to compete with staphylococci but also to acquire iron from S. aureus (Mashburn et al., 2005; Figure 2, left lower panel). Because freely available iron is often limited, P. aeruginosa has developed several strategies to scavenge iron, like the synthesis of iron chelating siderophores, pyoverdin, and pyochelin (Diggle et al., 2007). Transcription patterns of iron-regulated genes of P. aeruginosa in the presence of S. aureus in vivo are the same as in high-iron conditions in vitro, suggesting that S. aureus might be an iron source for P. aeruginosa (Mashburn et al., 2005). This type of interaction is, however, only important when both species are located close together, like in multi-species biofilms (Mashburn et al., 2005). The specificity of the LasA protease for staphylococci makes it a potential therapeutic candidate against staphylococcal infections especially those cause by antibiotic resistant MRSA strains as shown in a rat model of endophthalmitis (Barequet et al., 2009). However, S. aureus can survive LasA by the emergence of L-form-like colonies, which lack a cell wall (Falcon et al., 1989), although the role of L-form like colonies in disease remains rather vague.

Interspecies competition in biofilms not only occurs by inhibiting or killing the other species but also by inducing its dispersal. Biofilm dispersal is mainly induced when the environment becomes less favorable, like in case of nutrient depletion, and is extensively reviewed by Petrova and Sauer (2016). The exact mechanisms that induce biofilm dispersal are currently unknown, although several factors have been investigated (Hall-Stoodley and Stoodley, 2005; Davies and Marques, 2009). Since most bacteria reside in a biofilm consisting of multiple species in vivo, the dispersal signal must be recognized by a wide range of species (Davies and Marques, 2009). One class of such molecules are the cis-monosaturated fatty acids, which are small extracellular messenger molecules with broad inter-phylum and even inter-kingdom activities (Davies and Marques, 2009). P. aeruginosa produces cis-2-decenoic acid, which induces a dispersion response in biofilms formed by a range of Gram-negative and Gram-positive bacteria, including S. aureus (Figure 2, left upper panel), yeast as well as in P. aeruginosa (Davies and Marques, 2009). Interestingly, Davies et al showed that dispersion was only induced when the microcolonies reached a minimum of 40 μm diameter and 10 μm of thickness, indicating that a certain threshold concentration is needed for cis-2-decenoic acid to become active (Davies and Marques, 2009). This molecule could possibly be employed to disrupt biofilms on surfaces, followed by disinfectants that can successfully clear planktonic bacteria.

Most P. aeruginosa strains produce rhamnolipids, biosurfactants consisting of one or two rhamnose molecules linked to one or two fatty acids (Soberon-Chavez et al., 2005). While cis-2-decenoic acid is mainly used as a common signal for dispersion at the final biofilm stages, rhamnolipids are used to dislodge competing bacteria from the biofilm. Many different rhamnolipid homologs are produced, depending on the Pseudomonas strain and carbon source, and their synthesis is quorum-sensing regulated (Soberon-Chavez et al., 2005). Rhamnolipids were shown to reduce the surface tension and to have an anti-adhesive and antimicrobial effect on many micro-organisms (Haba et al., 2003; Rodrigues et al., 2006; Zezzi do Valle Gomes and Nitschke, 2012). The amphiphilic nature of rhamnolipids enables them to intercalate into the cell membranes of different microorganisms and form complexes, thereby permeabilizing the membranes and causing leakage of intracellular material (Sotirova et al., 2008). Gram-positive organisms seem more susceptible to rhamnolipid permeabilization than Gram-negative because the presence of lipopolysaccharides protects the cell membranes of the latter against the effect of surfactants (Soberon-Chavez et al., 2005). Furthermore, rhamnolipids were shown to promote biofilm dispersal in many different microorganisms, including P. aeruginosa itself, although this effect is strain dependent (Rodrigues et al., 2006). For S. aureus and S. epidermidis, rhamnolipids were shown to induce biofilm dispersal and to inhibit adhesion in a dose-dependent manner (Rodrigues et al., 2006; Zezzi do Valle Gomes and Nitschke, 2012; Pihl et al., 2013; Figure 2, left upper panel).

The N-acylhomoserine lactone (AHL) system is the most important and most extensively studied quorum-sensing system in P. aeruginosa. Many diverse AHLs are produced by various Gram-negative bacteria, all consisting of a homoserine lactone ring that is N-acylated with a fatty acyl group (Jimenez et al., 2012). The length of the acyl chains may vary from 4 to 18 carbons and P. aeruginosa mainly produces a short-chain C4-HSL and a long-chain 3-oxo-C12-HSL, although other lengths might also occur (Jimenez et al., 2012). The production of many virulence factors is regulated by AHL, including that of pyocyanin and rhamnolipids (Jimenez et al., 2012). Although it is currently not described that Gram-positive bacteria produce AHLs, they might still be influenced by them (Qazi et al., 2006). For example, growth of S. aureus is inhibited by several long chain 3-oxo-AHLs (including C8, C10, C12, and C14 chains) in a concentration dependent manner, with C12 and C14 being the most effective (Qazi et al., 2006). At concentrations below growth inhibition, the function of staphylococcal accessory regulator sarA and accessory gene regulator agr are strongly reduced (see Section SCV Induction by P. aeruginosa is Sigma B Dependent), and consequently their dependent virulence factors like hemolysins, TSST-1, protein A, and fibronectin-binding proteins (Figure 2, right lower panel; Qazi et al., 2006). Moreover, inhibition of agr might lead to more biofilm formation in S. aureus due to reduced detachment, although this study only tested planktonic conditions and requires further research (Qazi et al., 2006). The short chain AHL produced by P. aeruginosa seems to have no effect on growth and agr expression of S. aureus (Qazi et al., 2006). Furthermore, long-chain AHLs produced by other Gram-negative bacteria are likely to have similar effects in S. aureus, although, again, more studies are required here.

As described before, wounds infected with both S. aureus and P. aeruginosa generally show delayed closure compared to the single species infected wounds (Dalton et al., 2011; Seth et al., 2012). In addition to host related factors, one of the reasons for this phenomenon might be the upregulation of S. aureus virulence factors during co-infection, as was demonstrated for the MRSA strain USA300 (Pastar et al., 2013). Interestingly, although the growth of USA300 was strongly inhibited by P. aeruginosa in vitro, this effect was much weaker in vivo (Pastar et al., 2013). Furthermore, co-infection of wounds in a pig model induced S. aureus virulence factors hla and pvl, encoding α-hemolysin and Panton-Valentine leucocidin (Figure 2, right lower panel; Pastar et al., 2013). Another example of increased virulence of S. aureus in the presence of P. aeruginosa is the induction of staphyloxanthin production observed in a white S. aureus variant isolated from a soft tissue wound (Antonic et al., 2013). This strain possessed an intact and functional crtOPQMN operon, which is essential for production of the staphyloxanthin pigment, but was unable to induce pigment production on its own. Interestingly, staphyloxanthin production was induced by a P. aeruginosa co-isolate (Antonic et al., 2013). Furthermore, the pigment production in a characteristically golden-yellow S. aureus strain, which was co-isolated with the Pseudomonas strain and the white S. aureus variant, remained unaffected. However, this contradicts other studies that report an inhibition of S. aureus pigment production by pyocyanin and pyoverdin produced by P. aeruginosa (Biswas et al., 2009). Another result of this study (Antonic et al., 2013) that is in contradiction with other studies is that there was an unchanged expression of sigB that encodes the alternative transcription factor sigma B and which is previously reported to be upregulated in the presence of P. aeruginosa (Mitchell et al., 2010). The discrepancies in results between different studies indicate the importance of co-evolution and adaptation of the different isolates to each other and their environment. Adaptation of S. aureus to P. aeruginosa might lead to an expression pattern that is similar to a stress-resistant phenotype.

After the induction of the SCV phenotype during exposure to HQNO, the expression of three main regulatory mechanisms of virulence and biofilm formation is altered. First, the alternative transcription factor sigma B (SigB) is upregulated (Mitchell et al., 2010). SigB regulates the general stress response of Gram-positive bacteria, repressing the expression of most exoenzymes and toxins, stimulating the expression of adhesins and promoting the persistence of S. aureus in host cells (van Schaik and Abee, 2005; Atalla et al., 2011). Second, stimulation of SigB was shown to stimulate the expression of the Staphylococcal accessory regulator SarA, which modulates the expression of the pore-forming toxin α-hemolysin (hla) and increases biofilm formation (Valle et al., 2003; Oscarsson et al., 2006; Mitchell et al., 2010). In addition, SigB represses a third important regulator of S. aureus biofilm formation, the accessory gene regulator (agr) system, which induces biofilm dispersal thereby decreasing the total biomass and increases the expression of hla (Boles and Horswill, 2008; Mitchell et al., 2010; Atalla et al., 2011). In conclusion, HQNO reduces the production of the toxin hla by increasing the expression of SigB, leading to an increased expression of SarA and a decreased expression of agr. Reduced expression of toxins helps S. aureus to remain intracellular and thus increase its chances of survival in the human host. The stimulation of sarA by upregulation of SigB might be counteracted by long chain AHLs, which inhibit sarA (Figure 2, right lower panel). The net effect is probably dependent on the Pseudomonas strain involved since the production of both factors might be variable between isolates (Qazi et al., 2006; Fugère et al., 2014).

P. aeruginosa produces three main exopolysaccharides (EPS): alginate, Pel, and Psl, which form the extracellular matrix in the biofilm and have a structural and protective function (Leid et al., 2005; Ryder et al., 2007; Colvin et al., 2011, 2012). Pel and Psl are the main EPS in non-mucoid strains (Colvin et al., 2012). The Psl polysaccharide is recently identified as repeating units of glucose-, mannose-, and rhamnose-sugars and is mainly produced during the attachment phase of the biofilm (Colvin et al., 2012). Psl-positive strains have an elastic matrix with highly effective cross-linking of the matrix components (Chew et al., 2014). Pel is glucose-rich and is mainly involved in pellicle formation and later stages of biofilm formation (Colvin et al., 2012). Contrary to Psl, Pel dominant strains form loose biofilm structures since Pel reduces the effective cross-linking in the matrix network (Figure 3; Chew et al., 2014). Consequently, Pel-mediated loosening of the P. aeruginosa biofilm allows S. aureus to infiltrate into the biofilm and form multi-species biofilms (Chew et al., 2014). In contrast, the role of Psl in multi-species biofilm formation is not very clear. Chew et al showed that co-culture of a Psl-positive strain (PAO1) and S. aureus resulted in separated microcolonies without much association between both species (Chew et al., 2014). Billings et al., on the other hand, showed that S. aureus was incorporated in the air-liquid interface of a Psl producing P. aeruginosa biofilm. Both studies use the same biofilm assay and the same P. aeruginosa strains, PAO1 and mutants derived from PAO1, but different S. aureus strains. Because the biofilm structure and dual species interactions are dependent on both P. aeruginosa and the Staphylococcus strains, this might explain the discrepancy between the studies. Nonetheless, both studies concluded that the EPS provides protection against antibiotics to all inhabitants of the biofilm, even the non-producers, although the biofilm as a whole is weakened (Billings et al., 2013). More specifically, Psl functions as a protective barrier against the antibiotics colistin and polymyxin B, whereas Pel offers a protective barrier against aminoglycosides (Figure 3; Colvin et al., 2011; Billings et al., 2013). These findings indicate that a minimum amount of EPS per cell present in the biofilm is needed for optimal protection against antibiotics (Billings et al., 2013). These data suggests, if S. aureus is able to survive killing by P. aeruginosa and to co-exist in a multi-species biofilm, it benefits from the antimicrobial barrier formed by the P. aeruginosa matrix components. However, the third EPS, alginate, was not shown to have an effect on S. aureus and S. epidermidis biofilm formation. Alginate is mainly associated with chronic infections as its overproduction leads to the mucoid phenotype that frequently arises during long-term CF lung infection (Ryder et al., 2007). The switch to a mucoid phenotype contributes to the establishment of chronic colonization since alginate offers structural protection against uptake by macrophages and antimicrobials by forming a barrier limiting the penetration of antimicrobials, macrophages and macrophage-derived products, such as the pro-phagocytic cytokine IFN-γ (Leid et al., 2005; Ryder et al., 2007). Moreover, Leid et al. suggest that alginate might cause the transition from acute to chronic infection by limiting IFN-mediated clearance by macrophages, which is the main mechanism of bacterial clearance during acute infection (Leid et al., 2005).

In addition to EPS, other extracellular factors are important in dual species biofilm formation. Yang et al showed that only P. aeruginosa strains producing type IV pili co-aggregate with S. aureus in microcolonies (Yang et al., 2011). Type IV pili probably facilitate biofilm formation by binding to extracellular DNA (eDNA), which is derived from dead bacteria and part of the biofilm matrix (Yang et al., 2011). P. aeruginosa strains defective for the production of type IV pili or even treating the biofilm with DNAse I was shown to reduce the growth of mixed-species microcolonies (Yang et al., 2011). Furthermore, using a single S. aureus laboratory strain, Armbruster et al. showed inhibition of surface attachment of some P. aeruginosa clinical isolates due to the secretion of protein A (SpA) by S. aureus. SpA is a cell-wall associated extracellular adhesive protein of S. aureus that mediates biofilm formation and disrupts phagocytosis (by binding to the Fc portion of IgG antibodies) and its secretion was shown to be increased in artificial sputum (Armbruster et al., 2016). Secreted SpA was shown to specifically bind both Psl and type IV pili of P. aeruginosa, stressing the importance of these two molecules in multispecies interactions (Figure 3; Armbruster et al., 2016). In a Psl producing P. aeruginosa, all SpA seem to bind to the Psl, leaving the type IV pili free to mediate biofilm formation. In absence of Psl, SpA binds to the PilA component of type IV pili and inhibits adhesion of P. aeruginosa (Armbruster et al., 2016). Furthermore, SpA seems to protect P. aeruginosa from phagocytosis, as the Psl-SpA complex is no longer recognized by anti-Psl IgG antibodies. SpA can also bind to the Fc domain of anti-Psl IgG antibodies and prevent recognition by neutrophils (Figure 3; Armbruster et al., 2016).

The antistaphylococcal molecules produced by P. aeruginosa are also active against other staphylococci, including S. epidermidis, although some are more resistant to killing compared to S. aureus. P. aeruginosa was shown to effectively inhibit and disrupt established S. epidermidis biofilms and induce detachment without killing during dual species biofilm formation (Qin et al., 2009; Pihl et al., 2010a,b). After co-inoculation in equal proportions, P. aeruginosa and S. epidermidis could coexist for up to 18 h. After this time point, the S. epidermidis cells in the biofilm are lysed by P. aeruginosa (Pihl et al., 2010a). These data suggest that there are two stages in interactions between P. aeruginosa and S. epidermidis, the first includes the induction of detachment of viable S. epidermidis cells from the biofilm, while in the second stage cell lysis causes the total detachment (Pihl et al., 2010a). Similar to S. aureus, the effect of P. aeruginosa on S. epidermidis is strain dependent as some P. aeruginosa strains have a more pronounced effect on some S. epidermidis strains while others are more resistant to P. aeruginosa (Pihl et al., 2010b). Nevertheless, extracellular products that prevent initial attachment of some S. epidermidis strains to surfaces might be an interesting option for the development of coatings for indwelling medical devices, like peritoneal dialysis catheters (Pihl et al., 2013). Moreover, in this model, P. aeruginosa supernatant components replaced serum proteins on the catheter surface and reduced S. epidermidis attachment (Pihl et al., 2013). In addition, exposure of a S. epidermidis biofilm on a catheter to P. aeruginosa supernatant also caused dispersal of S. epidermidis (Pihl et al., 2013). The dispersed cells are, however, not killed making it less suitable as a treatment option and only interesting as a prevention strategy.

P. aeruginosa is originally an environmental bacterium and shares a niche with many other, non-pathogenic staphylococci like the Staphylococcus intermedius group consisting of S. delphini, S. intermedius, S. lutrae, S. pseudointermedius and S. schleiferi. All are common colonizers of various animals and rarely occur in humans (Simou et al., 2005; Ruscher et al., 2009). This group of staphylococci produces two low molecular weight compounds, yayurea A and B, that inhibit the production of quorum-sensing regulated products in Gram negative bacteria and provide protection against extracellular compounds produced by P. aeruginosa (Chu et al., 2013). For example, the growth of S. delphini is not suppressed by respiratory toxins during co-culture with P. aeruginosa. Moreover, S. delphini is able to completely inhibit the production of pyocyanin (Chu et al., 2013). The quorum-quenching effect of yayurea A and B covers a broad spectrum of Gram negative bacteria, including P. aeruginosa, Serratia marcescens, Vibrio harveyi, and Chromobacterium subtsugae (Chu et al., 2013). Quenching of the quorum-sensing system of these Gram negative bacteria does not kill them but rather maintains their physiological state as if the cell density is low, even though density is in fact high. This increases the chances of survival of the staphylococci since toxin production usually begins at high cell density (Chu et al., 2013). Surprisingly, other staphylococci seem to be resistant to both molecules, even though they are not producers (Chu et al., 2013). Interestingly, S. aureus is protected from killing by P. aeruginosa when yayurea A and B are added to the medium without the former having to undergo physiological changes (SCV formation), amd represent promising candidates for inhibition studies of P. aeruginosa virulence and biofilm formation.

In addition to the S. intermedius group, several other non-pathogenic staphylococci (S. carnosus, S. piscifermentans, and S. simulans) seem to be resistant to respiratory toxins secreted by P. aeruginosa due to alterations in the cydAB genes. These genes encode the two subunits of cytochrome bd quinol oxidase, of which homologs are also present in the genomes of S. aureus and S. epidermidis (Voggu et al., 2006). However, only the cytochrome bd quinol oxidase of the first group is resistant to the respiratory toxin, pyocyanin (Voggu et al., 2006). Furthermore, cloning of the S. carnosus cydAB cluster into S. aureus confers resistance to respiratory inhibitors produced by P. aeruginosa (Voggu et al., 2006). Further research showed that, whereas the CydA subunit is more conserved in staphylococci, CydB underwent a microevolution with relatively higher identity within than between the groups of pathogenic and non-pathogenic staphylococci (Voggu et al., 2006). This asymmetric evolution of CydB could be explained by the fact that the non-pathogenic staphylococci frequently inhabit the same environment as Pseudomonas spp. and were therefore selected for a higher resistance to respiratory toxins.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.