pathogenersis - lecture 4

pathogenesis of bacterial infection
postgraduate student – l – no. 3
terms frequently used in describing aspects of pathogenesis are defined in the glossary:
infection: multiplication or invasion of a pathogen within the body. multiplication of the bacteria that are part of the normal flora of the gastrointestinal tract, skin, etc, is generally not considered an infection on the other hand, multiplication of pathogenic bacteria (eg, salmonella species)—even if the person is asymptomatic—is deemed an infection.
pathogen: a microorganism capable of causing disease.
pathogenicity: the ability of an infectious agent to cause disease.
virulence: it is a measure of the severity of the disease it causes. virulent agents cause disease when introduced into the host in small numbers it is a quantitative ability of an agent to cause disease. for example, virulence can be quantified by how many microorganism are required to cause disease in fifty percent of those exposed to the pathogen (id50, where i= infectious and d= dose) or to kill fifty percent of test animals (ld50, where l= lethal, d= dose).
adherence (adhesion, attachment): the process by which bacteria stick to the surfaces of host cells. once bacteria have entered the body, adherence is a major initial step in the infection process.
invasion: the process whereby bacteria, parasites, fungi, and viruses enter host cells or tissues and spread in the body.
toxigenicity: the ability of a microorganism to produce a toxin that contributes to the development of disease.
carrier: a person or animal with asymptomatic infection that can be transmitted to another susceptible person or animal.
non pathogen: a microorganism that does not cause disease may be part of the normal flora.
opportunistic pathogen: an agent capable of causing disease only when the host s resistance is impaired (ie, when the patient is "immunocompromised").

bacterial pathogenesis
the pathogenesis of bacterial infection includes initiation of the infectious process and the mechanisms that lead to the development of signs and symptoms of disease. characteristics of bacteria that are pathogens include entrance, adherence to host cells, invasion of host cells damage of host cells by toxins (toxigenicity), and ability to evade the host s immune system. many infections caused by bacteria that are commonly considered to be pathogens are unapparent or asymptomatic. disease occurs if the bacteria or immunologic reactions to their presence cause sufficient harm to the person.


the infectious process can, in general be divided into several stages
1- entry into the host with evasion of host primary defenses
2- adhesion of microorganism to host cells
3- invasion or colonization of the organism to host cells
4- damage by bacterial toxins to host cells
5- evasion of host secondary defenses.
1-entry into the host with evasion of host primary defenses: it is the first step of infectious process is the entry of organism into the host by one of several port such as the respiratory tract, gastrointestinal tract, urogenital tract or skin that has cut, punctured, or burned. once entry is achieved, the pathogen must overcome a diversity of host defenses before can establish itself, this include phagocytosis, the acidic environment of the stomach and urogenital tract in addition to hydrolytic and proteolytic enzyme found in saliva, stomach and small intestine. bacteria that have an outer polysaccharide capsule for example neisseria meningitides and streptococcus pneumoniae have a better chance for evasion this primary defenses.
2- adhesion of microorganism to host cells
many adherence factors determine bacterial virulence or ability to cause infection and disease:
once bacteria enter the body of the host, they must adhere to cells of a tissue surface. if they did not adhere, they would be swept away by mucus and other fluids that bathe the tissue surface. adherence, which is only one step in the infectious process, is followed by development of microcolonies and subsequent steps in the pathogenesis of infection. the interactions between bacteria and tissue cell surfaces in the adhesion process are complex. several factors play important roles: a- surface hydropinghobicity and net surface charge, b- binding molecules on bacteria (ligands), and host cell receptor interactions.
different strains of bacteria within a species may vary widely in their hydropinghobic surface properties and ability to adhere to host cells. bacteria and host cells commonly have net negative surface charges and, therefore, repulsive electrostatic forces. these forces are overcome by hydropinghobic and other more specific interactions between bacteria and host cells. in general, the more hydropinghobic the bacterial cell surface, the greater the adherence to the host cell. bacteria also have specific surface molecules that interact with host cells.
many bacteria have pili, hair-like appendages that extend from the bacterial cell surface and help mediate adherence of the bacteria to host cell surfaces. for example, some e coli strains have type 1 pili, which adhere to epithelial cell receptors.
neisseria gonorrhoeae uses pili as primary adhesins and opacity associated proteins (opa) as secondary adhesins to host cells. certain opa proteins mediate adherence to polymorphonuclear cells and some gonococci survive after phagocytosis in these cells. pili and opa together enhance the invasion of cells cultured in vitro. group a streptococci (streptococcus pyogenes) also have hair-like appendages, termed fimbriae that extend from the cell surface. lipoteichoic acid, protein f, and m protein are found on the fimbriae. the lipoteichoic acid and protein f cause adherence of the streptococci to buccal epithelial cells this adherence is mediated by fibronectin, which acts as the host cell receptor molecule. m protein acts as an antiphagocytic molecule and is a major virulence factor. legionella pneumophila infects pulmonary macrophages and causes pneumonia. adherence of the legionellae to the macrophage induces formation of a long, thin pseudopod which then coils around the bacteria, forming a vesicle (coiling phagocytosis). the vesicle remains intact, phagolysosome fusion is inhibited, and the bacteria multiply within the vesicle.
3- invasiveness or colonization of the organism
invasive bacteria are those that can enter the host cells or penetrate mucosal surfaces, spreading from the initial site of infection. invasiveness is facilitated by several bacterial enzymes, the most notable of which are collagenase, hyaluronidase and coagulase. these enzymes degrade components of the extracellular matrix, providing the bacteria with easier access to host cell surfaces. invasion is followed by inflammation, which can be either pyogenic (involving pus mostly neutrophile) or granulomatus (having nodular inflammatory lesions contain fibroblasts, lymphocytes and macrophages).
for example clostridium perfringens produces the proteolytic enzyme collagenase, which degrades collagen, the major protein of fibrous connective tissue, and promotes spread of infection in tissue. hyaluronidases are enzymes that hydrolyze hyaluronic acid, a constituent of the ground substance of connective tissue. they are produced by many bacteria (eg, staphylococci, streptococci, and anaerobes) and aid in their spread through tissues.
s aureus produces coagulase, which works in conjunction with blood factors to coagulate plasma. coagulase contributes to the formation of fibrin walls around staphylococcal lesions, which helps them, persist in tissues. coagulase also causes deposition of fibrin on the surfaces of individual. staphylococci, which may help protect them from phagocytosis or from destruction within phagocytic cells.
4 damage by bacterial toxins to host cells
some bacteria cause disease by producing toxins produced are generally classified into two group’s extracellular toxin often called exotoxins and endotoxins as fallow:
a- exotoxins
many gram-positive and gram-negative bacteria produce exotoxins of considerable medical importance. clostridium diphtheriae is a gram-positive rod that can grow on the mucous membranes of the upper respiratory tract or in minor skin wounds. many factors regulate toxin production when the availability of inorganic iron is the factor limiting the growth rate, then maximal toxin production occurs. the toxin molecule is secreted as a single polypeptide molecule (mw 62,000). this native toxin is enzymatically degraded into two fragments, a and b, linked together by a disulfide bond. fragment b (mw 40,700) binds to specific host cell receptors and facilitates the entry of fragment a (mw 21,150) into the cytoplasm. fragment a inhibits peptide chain elongation factor ef-2 by catalyzing a reaction that yields free nicotinamide plus an inactive adenosine diphosphate-ribose-ef-2 complex. arrest of protein synthesis disrupts normal cellular physiologic functions. diphtheria toxin is very potent.
a

nad+
ef-2 ef-2 adpr


c. tetani is an anaerobic gram-positive rod that causes tetanus. clostridium tetani from the environment contaminates wounds and the spores germinate in the anaerobic environment of the injured tissue. infection often is minor and not clinically apparent. the vegetative forms of c. tetani produce the toxin tetanospasmin (mw 150,000) that is cleaved by a bacterial protease into two peptides (mw 50,000 and mw 100,000) linked by a disulfide bond. the tetanospasmin toxin acts on certain cells (interneurons) in central nervous system by inhibiting the release of neurotransmitter from these cells. tetanospasmin permits the simultaneous contraction of both muscles in a protagonist antagonist pair producing spastic (rigid) paralysis.
a

neurotransmitters toxin cleaves a protein involved in released of neutransmitter


b- endotoxin (lipopolysaccharides) of gram-negative bacteria
the lipopolysaccharides of gram-negative bacteria are derived from cell walls and are often liberated when the bacteria lyse. the substances are heat-stable, have molecular weights between 3000 and 5000 (lipooligosaccharides, los) and several million (lipopolysaccharides). they have three main regions (o- specific polysaccharide, core polysaccharide, lipid a)
lps in the bloodstream is initially bound to circulating proteins which then interact with receptors on macrophages and monocytes and other cells of the reticuloendothelial system. interleukin-1, tumor necrosis factor, and other cytokines are released, complement and coagulation cascades are activated this can be observed clinically or experimentally like fever, leukopenia, and hypoglycemia hypotension and shock resulting in impaired perfusion of essential organs (eg, brain, heart, kidney) intravascular coagulation and death from massive organ dysfunction. injection of lps produces fever after 60–90 minutes, the time needed for the body to release il-1. injection of il-1 produces fever within 30 minutes. repeated injection of il-1 produces the same fever response each time, but repeated injection of lps causes a steadily diminishing fever response because of tolerance due in part to reticuloendothelial blockade and in part to igm antibodies to lps.

table: characteristics of exotoxins and endotoxins (lipopolysaccharides)


exotoxins endotoxins
excreted by living cell high concentrations in liquid medium integral part of the cell wall of gram-negative bacteria. released on bacterial death and in part during growth. may not need to be released to have biologic activity
produced by both gram-positive and gram-negative bacteria found only in gram-negative bacteria
polypeptides with a molecular weight of 10,000–900,000 lipopolysaccharide complexes. lipid a portion probably responsible for toxicity
relatively unstable toxicity often destroyed rapidly by heating at temperatures above 60°c relatively stable withstand heating at temperatures above 60°c for hours without loss of toxicity
highly antigenic stimulate formation of high-titer antitoxin. antitoxin neutralizes toxin weakly immunogenic antibodies are antitoxic and protective. relationship between antibody titers and protection from disease is less clear than with exotoxins
converted to antigenic, nontoxic toxoids by formalin, acid, heat, etc. toxoids are used to immunize (eg, tetanus toxoid) not converted to toxoids
highly toxic fatal to animals in microgram quantities or less moderately toxic fatal for animals in tens to hundreds of micrograms
usually bind to specific receptors on cells specific receptors not found on cells
usually do not produce fever in the host usually produce fever in the host by release of interleukin-1 and other mediators
frequently controlled by extrachromosomal genes (eg, plasmids) synthesis directed by chromosomal genes


5- evasion of host secondary defenses (antigenic switching)
a successful pathogen must evade the host’s immune system that recognizes bacterial surface antigens. once important evasive strategy of the pathogen is to change it’s surface antigens, this is accomplished by several mechanisms. one mechanism, called phase variation, is the genetically reversible ability of certain bacteria to turn off and turn on the expression of genes coding for surface antigens. a second mechanism, called antigenic variation, involves the modification of the gene for an expressed surface antigen by genetic recombination with one of many variable unexpressed dna sequences.