Bacteria produce and sense a previously unsuspected diversity of small molecules used as chemical cues or signals to adapt to changing environments (Straight and Kolter, 2009). Bacterial communication via diffusible molecules has been extensively studied in the last decade, leading to active re-examination of the potential functions of secondary metabolites, a broad class of small molecules long considered as inessential by-products of the main metabolic reactions (Monds and O'Toole, 2008; Surette and Davies, 2008). Secondary metabolites are involved in a wide range of functions, from monitoring of cell density to growth co-ordination, virulence and other unexplored biological processes (Price-Whelan et al., 2006; Monds and O'Toole, 2008; Surette and Davies, 2008).
Recent investigations revealed that a neglected class of secondary metabolites corresponding to volatile compounds of fungi, plants, mammals, insects and nematodes could influence microbial activity and virulence (Cugini et al., 2007; Kai et al., 2009; Minerdi et al., 2009). Volatile compounds of bacterial origin were shown to contribute to cross-kingdom interactions, such as promotion of plant growth and defences or colonization of nematode hosts (Ryu et al., 2003; Farag et al., 2006; Niu et al., 2010).
However, volatile compounds have been studied primarily in a context of inter-kingdom responses and little is known about their potential roles in bacteria-to-bacteria interactions. Several studies suggested that volatile molecules emitted by bacteria could influence bacterial phenotypes such as colony morphogenesis, biofilm and pigment production (Kai et al., 2009; Cepl et al., 2010; Nijland and Burgess, 2010). In addition, an intriguing brief study reported that an unknown volatile compound produced by Escherichia coli could increase resistance to ampicillin and tetracycline antibiotics in physically separated E. coli recipient bacteria (Heal and Parsons, 2002).
Here, we investigated the molecular bases of the latter phenomenon using an original experimental design allowing remote aerial induction of antibiotic resistance. Using this phenotype as a read-out, we show that the implicated volatile compound is gaseous ammonia produced by the catabolism of l-aspartate in E. coli K12. When ammonia diffusing from bacterial cultures reaches a concentration threshold, its uptake via the E. coli Amt gas channel leads to increased levels of intracellular polyamines. The resulting changes in membrane permeability induce modifications of antibiotic sensitivity profiles and resistance to oxidative stress in physically distant bacterial cells. Although the involved molecular mechanisms may differ in other bacteria, we also show that exposure to ammonia produced by all tested microorganisms also induce non-inherited modification of the resistance profile to tetracycline, ampicillin and aminoglycosides in other aerially exposed bacteria, including Pseudomonas aeruginosa, Bacillus subtilis and Staphylococcus aureus. Our report therefore demonstrates that long-range bacteria-to-bacteria chemical interactions mediated by gaseous ammonia extend beyond its sole role as a nitrogen source.