Streptococcus pneumoniae is able to survive and thrive in human saliva, Dutch researchers have shown.
Their study identified certain genes that are involved in salivary S. pneumoniae survival and also suggests that the bacterium can use human saliva as a vector for transmission.
Hester Bootsma (Radboud University Medical Centre, Nijmegen, the Netherlands) and co-authors say that their finding “underscores that S. pneumoniae can be transmitted via respiratory droplets”.
The team examined the behaviour of several encapsulated and unencapsulated strains of S. pneumoniae when added to human saliva under varying experimental conditions.
At room temperature and in the absence of carbon dioxide (ie, conditions representing bacterial transmission), as well as 37°C and 5% carbon dioxide (ie, representing in-host carriage), all tested strains of S. pneumoniae survived and in some instances grew.
The degree of bacterial growth positively correlated with saliva concentration, being highest in undiluted (100%) saliva, report the authors. “This suggests that the pneumococcus is able to feed on and grow in human saliva,” they write.
Certain bacterial strains – namely, globally significant antimicrobial-resistant clones identified by the Pneumococcal Molecular Epidemiology Network (PMEN) – were particularly well-able to survive and grow in human saliva.
“It is tempting to speculate that the wide geographical distribution of the PMEN clones may be related to their increased ability to grow in human saliva and transmit through droplet spread,” write Bootsma et al. “The epidemiological spread of PMEN clones is especially worrisome because these clones consist of penicillin-resistant and multidrug-resistant pneumococci and thus contribute to the increase in antimicrobial resistance worldwide.”
Finally, the team used Tn-seq, a genome-wide screening technology, to identify 78 genes that are potentially needed for S. pneumoniae to survive in saliva without carbon dioxide. Three of these – the plsX gene and the amiACDEF and aroDEBC operons – were validated as playing an important role in the growth and survival of the bacteria.
These genes are involved in fatty acid metabolism, oligopeptide transport, and biosynthesis of aromatic amino acids, respectively.
Writing in PLoS ONE, the researchers say that their study has provided a genome-wide list of genes potentially involved in adaptation, and conclude: “The results described in this study provide a basis for future investigations characterizing the survival and pathogenesis of S. pneumoniae in human saliva.”
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