Pathogens often reduce their hosts' fitness. But in some systems, infected hosts sometimes do better than pathogen-free ones. CIDD researchers and collaborators have studied one such system in detail.
Bordetella bronchiseptica, a bacterium that causes respiratory disease in mammals, can be infected with the virus BPP-1, a bacteriophage. This phage can either incorporate its DNA into the bacterial genome, or replicate inside the bacterium before lysing (killing) it.
The researchers cultured bacterial strains that were genetically identical except that some strains carried the phage in their genomes and some contained antibiotic markers. Nutrient levels were high, so the bacteria did not compete for food for at least 24 hr. During this high-resource phase, all strains grew at the same exponential rate when cultured on their own. Phage-containing strains also grew exponentially when cultured in the presence of a phage-free strain and externally added phage. But when a phage-free strain was cultured with a phage-containing strain and externally added phage, it grew more slowly than when cultured on its own: while some of the phage-free bacteria incorporated the phage into their genomes, many were killed by it.
Using a combination of experimental data and mathematical analysis, the researchers showed that while the amount of free phage affected the transient dynamics of the two-strain system, it did not affect the steady-state outcome. Instead, the ultimate ratio of phage-containing to phage-free bacteria depended only on the initial ratio of the two strains and the probability that a bacterium of the sensitive strain survived infection.
Written By: Jaewook Joo, Michelle Gunny, Marisa Cases, Peter Hudson, Rн©ka Albert, & Eric Harvill
Journal: 273: 1843-1848
Journal Reference: 273: 1843-1848
Paper Id: 10.1098/rspb.2006.3512