Rise of the microbes

Abstract

Infectious diseases continue to plague the modern world. In the evolutionary arms race of pathogen emergence, the rules of engagement appear to have suddenly changed. Human activities have collided with nature to hasten the emergence of more potent pathogens from natural microbial populations. This is evident in recent infectious disease outbreaks, the events that led to their origin, and lessons learned: influenza (2009), meningitis (Africa, 2009), cholera (Haiti, 2010), E. coli (Germany, 2011) and Salmonella (USA, 2012). Developing a comprehensive control plan requires an understanding of the genetics, epidemiology and evolution of emergent pathogens for which humans have little or no pre-existing immunity. As we plot our next move, nature's genetic lottery continues, providing the fuel to transform the most unlikely infectious disease scenarios into reality.

Keywords: Salmonella; Shiga toxin-producing E. coli; cholera; emerging pathogens; hypervirulence; infectious disease outbreaks; influenza; meningitis; microbial pathogenesis; vaccines.

Figure 1. Hypervirulent Salmonella utilize a “Trojan Horse” strategy—exposing their virulence functions only during the infective process—but appearing much like other less-virulent strains in the environment. Entry into an animal host signals a dramatic shift in gene expression that is characterized by elevated toxin production coupled with the disruption of the host innate immune cytokine response necessary to execute antimicrobial activities. Exposure to ex vivo conditions signals a rapid transition to a less-virulent state characterized by more competitive growth in the environment. This rapid and reversible switching allows the bacterium to rapidly adapt to disparate hosts/environments without undergoing irreversible changes in the genome—providing a means for increased immune evasion/disruption, heightened disease and increased maintenance in nature. Hypervirulent Salmonella are among the most virulent of this species and are difficult to detect by current diagnostics. Credit: Peter Allen, UC Santa Barbara.