Nutrient Composition Promotes Switching between Pellicle and Bottom Biofilm in Salmonella

Abstract

<i>Salmonella</i> is one of the most frequently reported causes of foodborne illness worldwide. Non-typhoidal serovars cause gastroenteritis in humans. <i>Salmonella</i> can grow on surfaces forming biofilms, contributing to its persistence since biofilms are difficult to eradicate due to the high resistance to antimicrobials and disinfectants. It has been described that there are two crucial biofilm promoting factors in <i>Salmonella</i>: curli and cellulose. The expression of both factors is coordinately regulated by the transcriptional regulator CsgD. Most biofilm studies of <i>Salmonella</i> have been performed by growing bacteria in low osmolarity rich medium and low temperature (25°C). In such conditions, the biofilm is formed at the air-liquid interface (pellicle biofilm). Remarkably, when <i>Salmonella</i> grow in minimal medium, biofilm formation switches from the air-liquid interface to the solid-liquid interface (bottom biofilm). In this report, the switching between pellicle and bottom biofilm has been characterized. Our data indicate that curli, but not cellulose, is crucial for the formation of both kinds of biofilms. In minimal medium, conditions promoting formation of bottom biofilm, a high transcriptional expression of <i>csgD</i> and consequently of the genes involved in the synthesis of curli and cellulose was detected. The nutritional status of the cells seems to be pivotal for the spatial distribution of the biofilms formed. When bacteria is growing in minimal medium the addition of amino acids downregulates the expression of <i>csgB</i> and causes the switch between bottom and pellicle biofilm. The crosstalk between general metabolism and biofilm formation is also highlighted by the fact that the metabolic sensor cAMP modulates the type of biofilm generated by <i>Salmonella</i>. Moreover, cAMP regulates transcriptional expression of <i>csgD</i> and stimulates pellicle biofilm formation, suggesting that the physiological conditions define the type of biofilm formed by <i>Salmonella</i>. The consequences of the switching between pellicle and bottom biofilm during either infection or survival in natural environments remain undercover.