Historically, infections have been thought of as being caused by a single microbial agent, and while this is true for some instances of disease, in reality it is often more complicated. There is mounting evidence that polymicrobial infections are much more common than once thought and can lead to increased virulence of the infecting partners. Despite this, polymicrobial interactions remain an understudied area, particularly for the deadly pathogens Acinetobacter baumannii (Ab) and Klebsiella pneumoniae (Kp).
There is a wealth of information on Ab and Kp virulence and resistance factors in mono-infections, but previous studies have rarely focused on their interaction dynamics in co-culture. Co-infections of Ab and Kp can occur at rates of up to 40% of infections involving either species, and generally lead to worse outcomes for patients including increased mortality rates and increased resistance to antibiotics.
Utilising a strain pair of Ab (AB6870155) and Kp (KP6870155) co-isolated from a single lung infection, we investigated the co-culture dynamics between these species. In planktonic culture Kp was initially more abundant, however, both strains were able to persist in the population and relative abundances approached 1:1 over time. This was true in both rich lab media and a synthetic lung mimicking media, indicating that these strains can co-exist in culture without outcompeting one another.
Analysis of an array of over 190 carbon sources indicated that the Kp strain had a broader range of carbon source utilisation than Ab. This raised the question of whether Kp could cross-feed Ab, allowing Ab to grow in environments where it may not be able to otherwise. It was found that Kp could cross-feed by-products of sugar fermentation, ethanol and lactate, to Ab. Furthermore, cross-feeding extended to other strain pairs of Ab and Kp that were not co-isolates, albeit to varying extents.
These findings have implications for their co-persistence in infection and environmental settings. Developing a greater understanding of the conditions under which Ab and Kp may be dependent upon one another could allow the development of strategies that target and destabilise this interaction, potentially decreasing persistence and allowing eradication of both strains.