With Sleight of DNA, Pneumonia Bacterium Dodges Vaccines
By SINDYA N. BHANOO
Researchers from seven countries have collaborated to analyze how a single strain of Streptococcus pneumoniae bacteria has morphed over 30 years and spread across the world, in an attempt to overcome the development of antibiotics and vaccines....
In looking at more than 240 samples, they found that since 1984, when the strain was first identified in Spain, it has turned over about three-quarters of its genome.
Over time, the bacteria mutated to better resist antibiotics and vaccines. The researchers found that it underwent both recombination, in which the DNA shuffles around, and base substitutions, in which individual nucleic acids in a DNA sequence change....
From the study:
Rapid Pneumococcal Evolution in Response to Clinical Interventions
Nicholas J. Croucher et al.
Science 28 January 2011:
Vol. 331 no. 6016 pp. 430-434
Epidemiological studies of the naturally transformable bacterial pathogen Streptococcus pneumoniae have previously been confounded by high rates of recombination. Sequencing 240 isolates of the PMEN1 (Spain23F-1) multidrug-resistant lineage enabled base substitutions to be distinguished from polymorphisms arising through horizontal sequence transfer. More than 700 recombinations were detected, with genes encoding major antigens frequently affected. Among these were 10 capsule-switching events, one of which accompanied a population shift as vaccine-escape serotype 19A isolates emerged in the USA after the introduction of the conjugate polysaccharide vaccine. The evolution of resistance to fluoroquinolones, rifampicin, and macrolides was observed to occur on multiple occasions. This study details how genomic plasticity within lineages of recombinogenic bacteria can permit adaptation to clinical interventions over remarkably short time scales."
Additional excerpts from the paper itself:
“Following the introduction of a heptavalent conjugate polysaccharide vaccine [Prevnar] in many countries since 2000, which includes capsule type 23F as one of its seven antigens, a decrease in the frequency of serotype 23F invasive disease and carriage has been observed. However, this has been accompanied by a rise in disease caused by nonvaccine-serotype pneumococci, such as the multidrug-resistant serotype 19A strains now common in the USA.”
“The final non-European group consists of serotype 19A U.S. isolates. These all date from between 2005 and 2007 and are distinct from all other U.S. PMEN1 isolates, which have capsular types included in [the original Prevnar vaccine]. This is evidence of a shift in the PMEN1 population in the USA: Rather than a change in capsule type occurring among the resident population, it seems that it has been eliminated by the vaccine and replaced by a different subpopulation within the lineage that has expanded to fill the vacated niche….Hence, it appears that these changes in serotype after vaccine introduction result from an expansion of preexisting capsular variants, which were relatively uncommon and not part of the predominant population, and would have therefore been difficult to detect before the existence of the selection pressure exerted by the vaccine.”
“Over a few decades, this single pneumococcal lineage has acquired drug resistance and the ability to evade vaccine pressure multiple times, demonstrating the remarkable adaptability of recombinogenic bacteria such as the pneumococcus.”