Growth Dynamics of a 3-Species Mixed Culture after an Antibiotic Pulse – Analyzed on the Single Species Level

Investigation of growth kinetics of mixed cultures is rarely subject in microbiological research. A major reason is the lack of appropriate analytical tools for single species quantification. Nevertheless, however, there is a basic interest for a better understanding of growth characteristics in microbial mixed cultures in various research fields. We are investigating a mixed culture consisting of Pseudomonas aeruginosa, Burkholderia cepacia and Staphylococcus aureus as a model system. All three strains are well-known to be opportunistic infectants and can occur as mixed culture in the lung of cystic fibrosis patients. In combination with mathematical modelling we focus on identifying microbial interactions and analyzing the dynamics of the system. Experimental setup in shake flasks and bioreactor ensures well-defined and controlled cultivation conditions. Species specific cell enumeration is done via a quantitative T-RFLP method optimized for this model culture [1]. Here, we present the results of kinetic investigations regarding cell growth and cell lyses after antibiotic treatment of pure and mixed cultures with ceftazidime (30 mg/L). For statistical relevance each culture was set-up in three parallel batches and additionally each sample was taken in duplicates. We compared pure cultures with mixed cultures and cultures with and without addition of ceftazidime. In all cultures P.aeruginosa was the dominating organism due to its superior growth rate even in antibiotic-treated cultures. A time lag between antibiotic pulse and onset of bacteriostasis, respectively bacteriolysis was observed for all 3 species. The time lag of S.aureus and B.cepacia in pure cultures was reduced by approximately 1 h compared to the mixed cultures. Furthermore the antibiotic-induced lyses rate of P.aeruginosa was increased in the mixed culture compared to the pure culture. The reasons for both findings are likely to be some kind of inter-species effects but both stay unclear so far. [1]Schmidt, J.K. et al.: (2007) BiotechBioeng 96(4):738-75