U4 AOS2 Topic 6: Pathogen Evolution

Pathogens such as bacteria and viruses can evolve to become unaffected by antimicrobial agents. Pathogens can be killed by antimicrobial agents such as antibiotics to kill bacteria. Sometimes the bacteria become resistant to these antibiotics.

Antibiotic resistant bacteria:  

Antibiotic resistance is accredited as natural selection where antibiotic acts as selection pressure which selects only antibiotic-resistant bacteria. The development of antibiotic-resistant bacteria involves the following stages:

1.   Variation: bacteria in a bacterial population vary genetically and show different phenotypes such as antibiotic resistance and antibiotic susceptibility.

2.   Selection pressure: antibiotic acting as selection pressure selects only antibiotic-resistant bacteria which is “selective advantage” and kills antibiotic susceptible bacteria.

3.   Heritability: the alleles for antibiotic resistance can be transferred to other bacteria by a process of bacterial conjugation so that the allele frequency of antibiotic resistance increases in a gene pool of bacteria.

Antigenic drift and shift in viruses:

Production of effective vaccines against viruses has become difficult because viruses can change their surface antigens by the process of antigenic drift and shift so that the immunological memory cell cannot detect these antigens.

·        Antigenic drift involves the gradual changes in the genes of viruses which code for the surface antigens. These changes in the genes are due to mutations. The accumulation of these mutations over time can cause the formation of subtypes of viruses which can no longer be detected by immunological memory cells.

·        Antigenic shift involves the sudden change in genes of viruses which code for surface antigens. These changes are caused by the process of viral recombination where two different strains of viruses combine to form a different subtype of virus which can no longer be detected by immunological memory cells.