Virulence

Theobald Smith, an eminent American microbiologist, developed expression P = NV/R for the probability of developing a disease.   In this expression, P is the probability of an individual developing a disease and is proportional to the number (N) of pathogenic organisms encountered, and the virulence (V) of that organism and inversely proportional to the resistance (R) of the host.

Virulence may be described as the “intrinsic nastiness” of a pathogen, that is, its ability to avoid host defenses and cause damage to the host either locally, at the site of infection or at distant points through the production of circulating toxins or by stimulating host cytotoxic reactions. There are many classes of factors, which contribute to the virulence of a pathogen. These include:

1. factors related to attachment (pili, flagellae, adhesins) (Klemm and Schembri, 2000)
2. the glycocalyx and other surface components that prevent phagocytosis and attack by other host cellular immune responses (alginate, M-proteins)
3. invasins, or enzymes that damage host tissues and convert them into bacterial nutrients (Hemolysins, elastases, nucleases, etc.)
4. toxins (endotoxins and exotoxins which are either integral components of the bacterial cell wall or toxic materials secreted into body tissues, e.g. TSST-1, the toxic shock syndrome toxin) 

Abundant evidence now suggests that much of the virulence of pathogens is under Quorum Sensing control and that many factors related to virulence are initiated only after a certain critical mass of cells has become established.  These factors include at least some in each of the above categories.

When the human body recognizes the presence of a pathogen, a vigorous inflammatory response including neutrophile and protective macrophage mobilization and a vigorous immune response is the usual consequence. The strategy of biofilm producers is therefore one of stealth. Were the bacterium to liberate virulence factors early in the infection process, the body could respond with its entire armamentarium of defenses when the bacterium is in its most vulnerable state. By delaying the release of such factors until a sufficient population of cells exists and a thick EPS matrix is established, biofilms avoid the major challenges of the hosts response to infection.

Perhaps the best-studied biofilm pathogen is the opportunist Pseudomonas aeruginosa. This organism can prosper in a wide range of habitats including soil, plant surfaces, burn injured human tissues, wounds and the lungs of individual who have inherited the autosomal recessive hereditary disease cystic fibrosis. This ability to exist in such different environments is a consequence of the genetic versatility of this bacterium and its ability to respond to different environmental cues by producing gene products conducive to its survival in each habitat. These include, in the case of burns and lung infections, the production of virulence factors, which facilitate:

• adherence to tissues,
• protection from host defenses
• the ability to convert host resources to bacterial advantage
• the ability to convert host resources into toxins 

P. aeruginosa produces virulence factors in each of the catagories described above and although some of these may be constituative, many are under QS control (Passador and Iglewski, 1995).

Adherence and Attachment

Pseudomonas produces both pili (fimbriae), which serve as adhesins binding the cells to the cellular surface, and flagellae which enable the bacterium to closely approach the mucus membrane surface so that attachment can occur. The observation that mutant strains lacking either pili or flagella are greatly reduced in virulence supports the contention that these are significant virulence factors.

Protection and Bacterial Surface Components

Most strains of P. aeruginosa isolated from the lungs of chronically infected cystic fibrosis patients are of a mucoid phenotype, that is the cell masses are encased in a thick slimy coat of an exopolysaccharide called alginate. The importance of this mucoid phenotype in infection is suggested by experiments in which rats were infected with nonmucoid strains of the P. aeruginosa. In the tissues of the infected rats, the bacterium rapidly converted to a mucoid phenotype (phase variation).

Conversion of Host Resources and Invasins

Pseudomonas secretes a variety of enzymes the function of which is to degrade host tissues providing nutrient materials and breaching host defensive barriers enabling the spread of the pathogen form its initioal site of infection. Two of these are elastase A and elastase B, products of the LasA elastase and LasB elastase genes respectively. Elastin is a protein that accounts for as much as one third of the protein of lung tissues and elastase A and B are capable of degrading this material. Destruction of this protein caused by elastases in experimental subjects resulted in the production of hemorraghic lesions of the skin. Elastin is not a unique substrate, for these proteases, immunoglobins, complement proteins and lysozyme are also degraded reducing the effectiveness of host defense mechanisms.

Toxin Production

Pseudomonas aeruginosa produces a number of toxins including Exotoxin A, the product of the toxA gene. This toxin is physiologically related to the diphtheria toxin and inhibits protein synthesis.

With all of these virulence factors, why is it that Pseudomonas aeruginosa is capable of invading only hosts who have suffered some compromising disability such as a burn, wound, immune deficiency or hereditary predisposition? Passador and Iglewski attribute this to two intrinsic properties of this organism. “First, P. aeruginosa is not very successful at overcoming initial physical barriers such as skin and gaining access to host tissues. Second, when P. aeruginosa enters a host, it is not particularly effective at avoiding host defense mechanisms. In healthy individuals, the normal defenses of the human body are sufficient to prevent infection by P. aeruginosa” (Passador and Iglewski, 1995). So in the early stages of infection, P. aeruginosa “hides out” and thus fails to trigger the aggressive host responses typically initiated by more virulent pathogens. It is only when Pseudomonas reaches a critical mass or quorum that the virulence factors are expressed but that time the organism is capable of dealing with host defenses because it has by this time developed a fully capable biofilm.