The Best of Frenemies in Polymicrobial Infections
Like humans, microbes share environments, interact, and alter their behavior, depending on who’s around. When microbes interact, their behaviors can also affect their environment. Interactions strictly between the microbes can be beneficial for one (commensalism), beneficial for both (mutualism), or beneficial for one and detrimental to another (parasitism). But microbial relationships are complex and often defy the boundaries of these terms. Microbes are sometimes colloquially described as “friends,” “foes,” and even “frenemies” (“Frenemies are enemies who act like friends” — Gretchen Wieners, Mean Girls) in a commentary by Dr. Sophie Darch and Dr. Carolyn Ibberson. The word frenemy works especially well for those relationships that defy the boundaries of commensalism, mutualism, and parasitism. In fact, “frenemies” are a perfect description for some of the relationships between coexisting microbes during an infection. During infections that involve 2 microbial species (co-infections) or more (polymicrobial infections), microbes can interact with each other within the host environment.
The effect of a microbial infection on the host depends both on the identity of the microbes and how those microbes interact. When the presence of another species leads to interactions that exacerbate infection, this is called polymicrobial synergy. Polymicrobial antagonism is also possible, where the presence of another species in an infection leads to interactions that improve host infection outcomes.
The opportunistic bacterial pathogen Pseudomonas aeruginosa provides several examples of how a single species can interact with others to alter microbial behavior and host outcomes during an infection. P. aeruginosa is associated with many different infections, including cystic fibrosis lung infections, pneumonia, and soft tissue infections. The environment in which P. aeruginosa lives also supports other microbial life, and P. aeruginosa is a classic frenemy to its neighbors. Two well-studied P. aeruginosa relationships are with the bacteria Staphylococcus aureus and fungi Candida albicans.
The Semi-Mutualistic Relationship of P. aeruginosa-S. aureus
The opportunistic pathogen S. aureus resides on the skin and in the nasopharynx of humans but is often isolated from cystic fibrosis lung infections and chronic wound infections along with P. aeruginosa. When together (see Figure 1, right) , these 2 species act as frenemies, displaying both negative and positive interactions with each other.
The negative interactions between P. aeruginosa and S. aureus mainly arise from P. aeruginosa inhibiting or killing S. aureus, which P. aeruginosa does to retrieve the iron from S. aureus. P. aeruginosa senses when S. aureus is around by detecting parts of the S. aureus cell wall—its peptidoglycan. When researchers isolate bacteria after long-term infection and co-infection from the lungs of cystic fibrosis patients, P. aeruginosa is less able to inhibit the growth of S. aureus, suggesting that P. aeruginosa adapts to the host environment and that its relationship with S. aureus becomes less hostile. The relationship becomes more complex inside the host lung, however, because despite these in vitro interactions, P. aeruginosa remains the dominant pathogen in cystic fibrosis patients.
Let’s look at some friendly aspects of this relationship. Both bacterial species release molecules, some of which help members of their same species regulate behavior through quorum sensing, but can be detected by other species as well. By detecting each others’ molecules (much like eavesdropping on a conversation), both bacterial species change their behavior to benefit themselves. When it detects P. aeruginosa-produced molecules, S. aureus forms small-colony variants or more robust biofilms, both of which enhance S. aureus antibiotic resistance. Conversely, some S. aureus-produced molecules protect P. aeruginosa from phagocytosis and initiate small-colony variants that increases its antibiotic resistance. Overall, these interactions indicate a relatively friendly relationship that ultimately provides fitness benefits for both bacterial species.
How does the P. aeruginosa-S. aureus co-infection affect the host? As one might expect with increased drug resistance and decreased immune system susceptibility, the polymicrobial synergy in these co-infections leads to worse host outcomes. These co-infections can also result in slower wound healing and increased inflammation and lung damage in cystic fibrosis infections. Increased antibiotic resistance in both species can additionally make infection treatment more difficult.
The Mostly Commensalistic Relationship of P. aeruginosa-C. albicans
The fungus C. albicans is a member of the normal human flora in and on various body sites including the mouth, skin, gut, and female genitals. P. aeruginosa and C. albicans co-infections (see Figure 2, right) can occur in cystic fibrosis lung infections, bloodstream infections, and pneumonia. As with P. aeruginosa and S. aureus, P. aeruginosa and C. albicans are frenemies, interacting both positively and negatively with each other. The relationship had an added layer of complexity due the ability of C. albicans to grow as 2 different cell shapes, or morphologies, either yeast or hyphal cells.
P. aeruginosa and C. albicans display unfriendly behavior toward each other both in vitro and in vivo. Some negative interactions depend on the morphology of C. albicans. P. aeruginosa kills C. albicans hyphal cells, but not C. albicans yeast cells. But P. aeruginosa-produced molecules can also prevent C. albicans from switching from yeast to hyphae and forming biofilms. P. aeruginosa co-infection can also inhibit C. albicans growth in surgery patients. On the other hand, C. albicans can cause a decrease in P. aeruginosa virulence, which may benefit the infected human host, but may also facilitate clearance by the immune system, which doesn’t favor P. aeruginosa.
These microbial neighbors, however, can experience some fitness benefits in co-culture and co-infection. Both microbial species experience higher mutation rates when grown in co-culture; increased mutation rates increase the chance of developing an antibiotic resistant phenotype, providing a benefit to each organism. C. albicans also prevents P. aeruginosa phagocytosis by the immune system. These interactions, which benefit both P. aeruginosa and C. albicans, can change the infection experienced by the host.
In some cases, interactions between P. aeruginosa and C. albicans lead to polymicrobial antagonism and result in improved host outcomes. But this antagonism depends on which microbe gets there first and the time until the second microbe appears. For instance, early lung exposure to C. albicans can prime the immune system to clear P. aeruginosa, which leads to less lung damage. This represents the ideal case: microbial co-infection where each microbe experiences decreased virulence and increased susceptibility to immune clearance, boding well for patient outcome.
Unfortunately, the real world situation isn’t this straightforward. Other studies have demonstrated polymicrobial synergy in P. aeruginosa-C. albicans infections when C. albicans infects first (as was the case above). The presence of C. albicans can modulate the immune response, leading to higher host susceptibility to P. aeruginosa in the lung. Polymicrobial synergy also occurs when P. aeruginosa infects the skin, which results in higher host susceptibility to C. albicans. The P. aeruginosa-C. albicans relationship is further complicated by factors such as fungal morphology and infection site.
Microbial Relationship Status: It's Complicated
Looking at the P. aeruginosa‘s relationships with S. aureus and C. albicans indicates that P. aeruginosa has a tendency to make frenemies, displaying commensalistic, mutualistic, and parasitic behavior, depending on environment. Regardless of the relationships between P. aeruginosa, S. aureus, and C. albicans, unfortunately, together, these microbes can be enemies to the host. The complicated nature of these microbial relationships is a major reason why these polymicrobial infections are so difficult to resolve.