Virulence Factors: Biomedical Sciences

• Virulence factors are being presented as part of Biomedical Sciences by Ms P Lazarou at Eastman Dental Hospital Education Centre/LSBU.
• The GDC learning outcomes include explaining general and systemic diseases and their relevance to oral health.
• Further learning outcomes include the aetiology and pathogenesis of oral diseases as well as potential routes of transmission of infectious agents in dental practice.
• Intended learning outcomes include defining the terminology associated with virulence and virulence factors.
• Further intended learning outcomes include recalling and listing pathogens which infect the human body.
• Additional learning outcomes involve describing the various mechanisms that pathogens deploy to destroy, cause malfunction of host cells, and evade the immune response.

Terminology

• A pathogen is any microorganism able to cause disease in a host organism, and it is important to list pathogens, which could cause disease in humans.
• A primary pathogen is an environmental microbe able to cause disease in an otherwise healthy individual.
• An opportunistic pathogen is a member of the normal microbiota that only causes disease in an immunocompromised host.
• Pathogenicity is the ability of a pathogen to cause disease.
• Examples of pathogenic diseases include ear infections, pneumonia and food poisoning which can be caused by bacteria.
• Viruses can cause the common cold, influenza and AIDS.
• Fungi can cause athlete's foot and ringworm.
• Protists can cause Malaria.
• Virulence is the degree or intensity of pathogenicity.
• Virulence factors are mechanisms determining the degree to which a pathogen causes damage, invasion, and infectivity.
• A chain of infection includes agent, virulence, dose, exposure, and then susceptibility.
• Molecular Koch's postulates should be reviewed in microbiology sessions.
• Virulence factors allow a pathogen to outcompete host cells and resist their defenses.
• Pathogens require a suitable environment and nutrients to survive and also protection from harmful elements.
• Virulence factor mechanisms include adhesion, evasion of phagocytosis, immunoevasion, and immunosuppression.
• Further mechanisms include toxigenicity as well as enzymatic actions.
• Staphylococcus aureus produces toxins like TSST-1 and enterotoxins, immunoglobulin and protein A, micro capsules and also adhesions.

Pathogen Entry

• In pathogenic infection, a pathogen proceeds through exposure, adhesion, invasion, colonization, toxicity, and tissue damage and disease.

Adhesion

• Adhesion involves pili, fimbriae, capsules, and bacterial cell walls.
• Class IV pili have a twitching motility and are found at poles of bacilli
• Class IV pili enable gliding motion along the solid host cell surface.
• Pili retract and extend, allowing movement, and also allows bacteria to slingshot over the cellular surface.
• Cell walls of bacteria contain surface proteins/glycoproteins called adhesins, which bind to targeted receptor molecules on the host cell surface.
• This allows the bacteria to adhere closely to the host cell, thereby resisting physical removal and allowing colonization.
• Many bacteria use one or more adhesins to colonize host cells.
• Streptococcus pyogenes causes strep throat with Protein F for adhesion to respiratory epithelial cells.
• Streptococcus mutans causes dental caries with Adhesin P1 for adhesion to teeth.
• Neisseria gonorrhoeae causes Gonorrhea with Type IV pili for adhesion to urethral epithelial cells.
• Enterotoxigenic E. coli (ETEC) causes traveler's diarrhea with Type 1 fimbriae for adhesion to intestinal epithelial cells.
• Vibrio cholerae causes Cholera with N-methylphenylalanine pili for adhesion to the intestinal epithelial cells.
• Capsules are produced by some bacteria in order to aid in adhesion.
• Aid in immune evasion by preventing phagocytosis by cells of the immune system.
• Capsules prevent adhesion of antibodies and capsule size deters phagocytosis.
• Some pathogens produce virulence factors that enable evasion of the immune system.
• Some Streptococcus species contain M protein in Fimbriae, which alters the surface of the bacterium and inhibits phagocytosis.
• Mycolic acid (waxy substance) is produced in the cell wall of Mycobacterium tuberculosis; it acts as a protective coat which deters killing mechanisms when phagocytosed.

Exoenzymes and Toxins

• Pathogen entry, adhesion, and colonization follows a sequence of exposure, adhesion, invasion, colonization, toxicity, and tissue damage & disease.
• Exoenzymes are extracellular enzymes secreted by cells, which function outside of those cells.
• Exoenzymes invade host cells and deeper tissues, and they are specific to particular tissue structures.
• Exoenzymes enables invasion and supports own growth and defends against the immune system.
• Glycohydrolases degrade hyaluronic acid that cements cells together to promote spreading through tissues, such as Hyaluronidase S in Staphylococcus aureus.
• Nucleases degrade DNA released by dying cells (bacteria and host cells) that can trap the bacteria, thus promoting spread, such as DNAse produced by S. aureus.
• Phospholipases degrades phospholipid bilayer of host cells, causing cellular lysis, and degrade membrane of phagosomes to enable escape into the cytoplasm, such as Phospholipase C of Bacillus anthracis.
• Proteases degrades collagen in connective tissue to promote spread, such as Collagenase in Clostridium perfringens.
• Toxins are biological poisons produced by some pathogens that invade and damage tissues.
• Toxigenicity is the ability of a pathogen to produce toxins to cause damage to host cells.
• There are two types of toxins: endotoxins and exotoxins.
• Endotoxins are derived from gram-negative bacteria and stimulate a general systemic inflammatory response.
• Endotoxins remain stable at high temperatures, requiring heating at 121°C (250°F) for 45 minutes to inactivate.
• If endotoxin concentration is low, the host's inflammatory response against infection is effective.
• High levels of endotoxins can cause severe drop in blood pressure, multi-organ failure, and possibly death.
• Exotoxins are potent protein molecules produced by wide variety of pathogenic and mainly gram-positive bacteria/some gram negative bacteria.
• Each exotoxin targets specific receptors on specific cells, and damages those cells through individual molecular mechanisms.
• Exotoxins are inactivated by heat (>41°C/106°F) and low concentrations can be lethal.
• Exotoxins are grouped in 3 categories: intracellular targeting, membrane disrupting, and superantigens.
• Terminology to describe pathogens in the bloodstream usually ends in -aemia.
• Presence of bacteria in the blood is referred to as bacteraemia.
• Pus-forming bacteria in the blood is referred to as pyaemia.
• presence of toxins in the blood is referred to as toxaemia.
• Multiplying bacteria in the blood is referred to as septicaemia.
• Staphylococcus aureus produces exoenzyme coagulase stimulating fibrinogen-to-fibrin cascade, which enables bacteria to be coated by fibrin clots, preventing phagocytosis.
• Kinases stimulate digestion of fibrin clots depending on conditions as the pathogen needs to escape and spread from clot.
• Antigenic variation involves the alteration of surface proteins in order to avoid recognition by the host's immune response.
• Further protection against the immune system involves destruction of phagocytes with production of leukocidins.
• More defense mechanisms also include destruction of T lymphocytes.
• There is also Intracellular positioning: immune defenses are not able to reach them, and pathogens avoid killing methods within the phagocyte.
• Viral virulence is mediated by adhesins that are part of the viral capsid or membrane envelope.
• Viruses interact with specific cell receptors (tropism), such as spike protein hemagglutinin on influenza virus.
• Glycoprotein g20 is found on HIV.
• Antigenic variation occurs in some enveloped viruses.
• Antigenic drift involves point mutations causing minor changes in the spike proteins.
• Antigenic shift involves gene re-assortment resulting in major changes in spike proteins
• Porphyromonas gingivalis modifies and stimulates immune responses like cytokine secretion or inhibition in the periodontium.
• Gingipains (proteases-exoenzyme) produced by p.gingivalis break down structural proteins of the periodontium like collagen, elastin, fibronectin.
• Main virulence factors of Streptococcus Mutans associated with cariogenicity are Adhesion, acidogenicity, and acid tolerance.
• These factors work together to change the ecology of dental plaque.
• Higher numbers of S. mutans as well as other acid producing and acid tolerant bacterial species can be found in dental plaque.
• The pH drops quickly due to the increase in cariogenic bacteria when the available carbohydrate is being fermented.
• Susceptibility to enamel demineralization increases at this time of bacterial proliferation and volume of acid being produced.
• Herpes simplex virus hides from the immune system in neurons and non-neuronal cells, and may persist for many years.
• This emerges as a pathogenic form when immune resistance is low and presents clinically as herpes labialis (cold sore).
• Candida Albicans is the most common etiological factor of opportunistic human fungal infections.
• Candida Albicans Exoenzymes secrete aspartyl proteases that are also known as 'Sap Proteins'.
• Candida Albicans can develop adhesions and through pleomorphism can respond to changing environmental conditions/adapts to biological niches.
• Pathogens have and continue to evolve strategies through virulence factors to increase virulence and avoid immune responses.
• The constant interaction between the pathogen and host cells is a dynamic one, where the production of new virulence factors are countered by the host.
• Increase of cytotoxic T cells in the host enables the pathogens to produce 'escape mutations' in order to avoid being a future target.
• Pathogens will continue to evolve and emerge, and the most successful ones will being those which take advantage of the host without causing death.