Effects of oral microbes on biofilm formation by respiratory pathogens

Introduction: Ventilator-associated pneumonia (VAP) is the most frequently occurring infection in intensive care units and second amongst all hospital-acquired infections. The endotracheal tube (ETT) subverts the respiratory tract defence mechanisms and provides a surface for biofilm growth. We hypothesise that oral microorganisms contribute to this ETT biofilm which facilitates subsequent colonisation by respiratory pathogens. Objectives:To characterise mixed species biofilms produced in vitro on ETT surfaces and establish whether oral microorganisms enhanced the presence of specific respiratory pathogens. Methods: ETT sections (1 cm) were added to standard inocula of different combinations of oral microorganisms (Streptococcus mutans, Porphyromonas gingivalis and Candida albicans) and recognized respiratory pathogens (Staphylococcus aureus, Pseudomonas aeruginosa). Biofilms were generated over a maximum of 10-days under both aerobic and anaerobic conditions at 37°C. At 24 h intervals the culture medium was replenished and at the end of incubation the biofilm was removed and species presence was assessed qualitatively and quantitatively by viable cell culture. Results: In single species preparations, all tested microorganisms were found to generate ETT biofilms. In terms of cell numbers, the most extensive biofilms were produced by S. mutans (4.2 x 108 cfu/cm2) followed by P. aeruginosa (6.5x107 cfu/cm2). All species were subsequently recovered from mixed species biofilms where S. mutans (9.1x107 cfu/cm2) and P aeruginosa (4.5x106 cfu/cm2) had highest prevalence. Importantly, compared with single species biofilms higher counts of S. aureus (8x106 cfu/cm2 compared to 3.3x105 cfu/cm2) and P. aeruginosa (1.7x109 cfu/cm2 compared to 6.5x107 cfu/cm2) occurred in dual preparations with S. mutans. Conclusion:Oral microbes such as S. mutans can have a synergistic effect on the growth of respiratory pathogens in these in vitro biofilm models. This is clinically important when developing strategies aimed at reducing the prevalence of VAP and highlights the potential benefit of maintaining oral care within intubated patients