Strain Background, Species Frequency, and Environmental Conditions Are Important in Determining Pseudomonas aeruginosa and Staphylococcus aureus Population Dynamics and Species Coexistence

Abstract

<jats:p>Bacterial communities in the environment and in infections are typically diverse, yet we know little about the factors that determine interspecies interactions. Here, we apply concepts from ecological theory to understand how biotic and abiotic factors affect interaction patterns between the two opportunistic human pathogens <jats:italic>Pseudomonas aeruginosa</jats:italic> and <jats:italic>Staphyloccocus aureus</jats:italic>, which often co-occur in polymicrobial infections. Specifically, we conducted a series of short- and long-term competition experiments between <jats:italic>P. aeruginosa</jats:italic> PAO1 (as our reference strain) and three different <jats:italic>S. aureus</jats:italic> strains (Cowan I, 6850, JE2) at three starting frequencies and under three environmental (culturing) conditions. We found that the competitive ability of <jats:italic>P. aeruginosa</jats:italic> strongly depended on the strain background of <jats:italic>S. aureus</jats:italic>, whereby <jats:italic>P. aeruginosa</jats:italic> dominated against Cowan I and 6850, but not against JE2. In the latter case, both species could end up as winners depending on conditions. Specifically, we observed strong frequency-dependent fitness patterns, including positive frequency dependence, where <jats:italic>P. aeruginosa</jats:italic> could dominate JE2 only when common, but not when rare. Finally, changes in environmental (culturing) conditions fundamentally altered the competitive balance between the two species, in a way that <jats:italic>P. aeruginosa</jats:italic> dominance increased when moving from shaken to static environments. Altogether, our results highlight that ecological details can have profound effects on the competitive dynamics between co-infecting pathogens, and determine whether two species can co-exist or invade each others' populations from rare. Moreover, our findings might parallel certain dynamics observed in chronic polymicrobial infections.</jats:p> <jats:p><jats:bold>Importance</jats:bold> Bacterial infections are frequently caused by more than one species and such polymicrobial infections are often considered more virulent and more difficult to treat than the respective monospecies infections. <jats:italic>Pseudomonas aeruginosa</jats:italic> and <jats:italic>Staphyloccocus aureus</jats:italic> are among the most important pathogens in polymicrobial infections and their co-occurrence is linked to worse disease outcome. There is great interest in understanding how these two species interact with each other and what the consequences for the host are. While previous studies have mainly looked at molecular mechanisms implicated in interactions between <jats:italic>P. aeruginosa</jats:italic> and <jats:italic>S. aureus</jats:italic>, here we show that ecological factors such as strain background, species frequency and environmental conditions are important elements determining population dynamics and species co-existence patterns. We propose that the uncovered principles may also play major roles in infections, and therefore proclaim that an integrative approach combining molecular and ecological aspects is required to fully understand polymicrobial infections.</jats:p>

A Pseudomonas aeruginosa Antimicrobial Affects the Biogeography but Not Fitness of Staphylococcus aureus during Coculture

“…Our findings that ecological factors (well-mixed and static growth) affect P. aeruginosa antistaphylococcal activity and the outcome of P. aeruginosa - S. aureus coculture dynamics are fundamentally in agreement with recent work from Niggli and Kummerli ( 20 ), which showed that P. aeruginosa and S. aureus coexist in a laboratory medium that promotes aggregation by embedding in low levels of agar. However, that study did not observe an increase in P. aeruginosa relative fitness compared to S. aureus during growth in mixed conditions compared to agar conditions, which differs from our observations ( Fig.…”

“…1A and B ). The reason for this is not clear, but it is likely explained by low aeration of the mixed culture condition used by Niggli and Kummerli ( 20 ). That study performed mixed coculture in laboratory medium in wells of 24-well plates, using a volume of 1.5 ml in a well with a maximum volume of 3.4 ml and shaking at 170 rpm ( 20 ).…”

“…The reason for this is not clear, but it is likely explained by low aeration of the mixed culture condition used by Niggli and Kummerli ( 20 ). That study performed mixed coculture in laboratory medium in wells of 24-well plates, using a volume of 1.5 ml in a well with a maximum volume of 3.4 ml and shaking at 170 rpm ( 20 ). In our experience, P. aeruginosa requires high shaking rates (250 rpm) and low culture volume/culture vessel volume (1/10 to 1/50) for sufficient aeration, and these culture conditions lead to high antistaphylococcal activity ( 15 , 19 , 21 , 22 ).…”

“…While our study focused on laboratory strains of P. aeruginosa and S. aureus , it is clear that the genotype of the strains used can impact relative fitness during coculture ( 20 , 40 , 57 ). Although we anticipate that relative fitness might be impacted by the use of other strains, as P. aeruginosa PA14 is highly lytic for S. aureus LAC under well-mixed conditions, their survival in static SCFM2 indicates that this model will likely promote coexistence for multiple genotypes, even those that are highly antagonistic.…”

“…Several studies have developed such systems by altering the bacterial genotype or growth conditions. For example, in vitro coexistence has been obtained by using a mucoid strain of P. aeruginosa that has less lytic activity against S. aureus ( 40 ), by exchanging medium and removing planktonic cells to extend coexistence ( 33 ), or by altering the frequency of P. aeruginosa / S. aureus or the growth environment ( 20 , 41 ). However, laboratory coculture models can be further improved with the explicit goal of mimicking both the chemical and physical environment of the human infection site ( 2 , 42 ).…”