High levels of cyclic di-GMP in Klebsiella pneumoniae attenuate virulence in the lung

The bacterial second messenger bis-(3=-5=)-cyclic dimeric GMP (c-di- GMP) has been shown to influence the expression of virulence factors in certain pathogenic bacteria, but little is known about its activity in the increasingly antibiotic-resistant pathogen Klebsiella pneumoniae. Here, the expression in K. pneumoniae of a heterologous diguanylate cyclase increased the bacterial c-di-GMP concentration and attenuated pathogenesis in murine pneumonia. This attenuation remained evident in mice lacking the c-di-GMP sensor STING, indicating that the high c-di-GMP concentration exerted its influence not on host responses but on bacterial physiology. While serum resistance and capsule expression were unaffected by the increased c-di-GMP concentration, both type 3 and type 1 pili were strongly upregulated. Importantly, attenuation of K. pneumoniae virulence by high c-di-GMP levels was abrogated when type 1 pilus expression was silenced. We conclude that increased type 1 piliation may hamper K. pneumoniae virulence in the respiratory tract and that c-di-GMP signaling represents a potential therapeutic target for antibioticresistant K. pneumoniae in this niche.