Bacteriophage typing is a phenotypic method used to distinguish strains of bacteria based on their susceptibility to lysis by a defined set of bacteriophages. Patterns of lysis provide a typing code that can be used for epidemiological tracking and strain identification.
Explanation
In bacteriophage typing, panels of bacteriophages with known host ranges are applied to lawns of the bacterial isolate on agar plates. Each phage produces a clear zone of lysis (plaque) on susceptible strains but not on resistant ones. The resulting pattern of plaques, recorded as a phage type, allows differentiation between strains of the same species. The technique has historically been important for tracing sources of infections caused by Staphylococcus aureus, Salmonella enterica serovars Typhi and Enteritidis, Vibrio cholerae and other pathogens. Because host range is determined by specific phage receptors on bacterial surfaces, small genetic changes in the bacterium can alter susceptibility, providing high discriminatory power. However, phage typing requires maintaining phage panels and may be influenced by spontaneous phage resistance. Molecular methods such as pulsed‑field gel electrophoresis and whole‑genome sequencing have largely superseded phage typing for routine surveillance, but it remains useful in some regions and in phage therapy to select lytic phages for individual infections.
Applications and examples
Classical phage typing of Staphylococcus aureus uses a set of 23 phages to assign types such as 52/52A/80/81, which were associated with hospital outbreaks in the mid‑20th century. Epidemiologists have used phage typing to link cases of typhoid fever to a common source by identifying isolates with the same Salmonella Typhi phage type. Phage typing of Salmonella Enteritidis and Salmonella Typhimurium has been integral to food safety investigations. For Vibrio cholerae, phage typing has distinguished El Tor and classical biotypes. In experimental therapy, lytic phage cocktails are tailored to patient isolates using spot tests that resemble phage typing.
Although molecular typing now provides more detailed resolution, bacteriophage typing illustrates the interplay between bacteria and their viruses and has contributed to our understanding of bacterial epidemiology.
Related Terms: Bacteriophage, Typing, Epidemiology, Host range, Molecular typing