Characteristics of successful pathogens

Characteristics of successful pathogens - Successful pathogens have a number of attributes. They compete with host cells and colonising flora by various methods including sequestration of nutrients, use of metabolic pathways not used by competing bacteria, or production of bacteriocins (small antimicrobial peptides proteins that kill closely related bacteria). Motility enables pathogens to reach their site of infection, often in sterile sites that colonising bacteria do not reach, such as the distal airway. Many microorganisms, including viruses, use ‘adhesins’ to attach to host cells at the site of infection. Other pathogens can invade through tissues.

Five pathogens were selected for testing in the prototype POCT
instrument: (a) Streptococcus Pneumonia, (b) Pseudomonas,
(c) Cadida, (d) Staphylococcus aureus, and (e) Escherichia coli.
Infections with any of these pathogens can lead to sepsis.
If a patient does not receive medical intervention within hours,
 death is imminent.

Pathogens

Pathogens may produce toxins, microbial molecules that cause adverse effects on host cells either at the site of infection or remotely following carriage through the blood stream. Endotoxin is a cell wall component released mainly following bacterial cell damage and has generalised inflammatory effects. Exotoxins are proteins released by living bacteria, which often have specific effects on target organs (Box 6.3.) Intracellular pathogens, including viruses, bacteria (e.g. Salmonella spp., Listeria monocytogenes and Mycobacterium tuberculosis), parasites (e.g. Leishmania spp.) and fungi (e.g. Histoplasma capsulatum), have the capacity to survive in intracellular environments, including after phagocytosis by macrophages. Pathogenic bacteria express different arrays of genes, depending on environmental stress (pH, iron starvation, O2 starvation etc.) and anatomical location. In quorum sensing, bacteria communicate with one another to adapt their replication or metabolism according to local population density. Bacteria and fungi may respond to the presence of an artificial surface (e.g. prosthetic device, venous catheter) by forming a biofilm, which is a population of organisms encased in a matrix of extracellular molecules. Biofilm-associated organisms are highly resistant to antimicrobial agents.

Genetic diversity enhances the pathogenic capacity of bacteria. Some virulence factor genes are found on plasmids or in phages and are exchanged between different strains or species. The ability to acquire genes from the gene pool of all strains of the species (the ‘bacterial supragenome’) increases diversity and the potential for pathogenicity. Viruses exploit their rapid reproduction and potential to exchange nucleic acid with host cells to enhance diversity. Once a strain acquires a particularly effective combination of virulence genes, it may become an epidemic strain, accounting for a large subset of infections in a particular region. This phenomenon accounts for influenza pandemics.