Yersinia pestis biofilm in the flea vector and its role in the transmission of plague

Abstract

Transmission by fleabite is a relatively recent evolutionary adaptation of Yersinia pestis, the bacterial agent of bubonic plague. To produce a transmissible infection, Y. pestis grows as an attached biofilm in the foregut of the flea vector. Biofilm formation both in the flea foregut and in vitro is dependent on an extracellular matrix (ECM) synthesized by the Yersinia hms gene products. The hms genes are similar to the pga and ica genes of Escherichia coli and Staphylococcus epidermidis, respectively, that act to synthesize a poly-beta-1,6-N-acetyl-d-glucosamine ECM required for biofilm formation. As with extracellular polysaccharide production in many other bacteria, synthesis of the Hms-dependent ECM is controlled by intracellular levels of cyclic-di-GMP. Yersinia pseudotuberculosis, the food- and water-borne enteric pathogen from which Y. pestis evolved recently, possesses identical hms genes and can form biofilm in vitro but not in the flea. The genetic changes in Y. pestis that resulted in adapting biofilm-forming capability to the flea gut environment, a critical step in the evolution of vector-borne transmission, have yet to be identified. During a flea bite, Y. pestis is regurgitated into the dermis in a unique biofilm phenotype, and this has implications for the initial interaction with the mammalian innate immune response.

Fig. 1

Y. pestis biofilm in the flea. Digestive tract dissected from an X. cheopis flea blocked with a dense biofilm consisting of dark masses of Y. pestis embedded in a paler, viscous ECM (arrows). The contiguous biofilm fills the proventriculus (PV) and extends posteriorly into the lumen of the midgut (MG). E, esophagus

Fig. 2

ECM surrounding Hms⁺ (a) but not Hms⁻ (b) Y. pestis grown on agar plates at 21 °C. Scale bar, 1 μm

Fig. 3

Biofilm produced on the glass surface of a flow cell after 48 h at 21°C by Hms⁺ (a) but not Hms⁻ (b) Y. pestis

Fig. 4

Y. pestis biofilm (arrowheads) expelled through the esophagus (E) by application of a cover slip to the digestive tract dissected from a blocked X. cheopis flea. The array of spines that line the interior of the proventriculus (PV) is visible