Normal flora and host parasite relationship MBBS- Batch 17 Remya

Learning objectives: The objectives of this lecture are to: define normal, resident and transient flora define to bring out the meaning of commensals , mutualism and parasitism. list the human factors that determine normal flora describe the nature of the microbiota on the various sites of the body discuss the impact of broad-spectrum antibiotics on the normal flora explain the useful as well as the harmful effects of the normal microbial biota on the human body

Learning outcomes: At the end of the lecture, students should be able to: describe the factors that effect and determine the normal flora. distinguish between commensalisms , mutualism and parasitism. identify the common flora residing at different sites. relate the effects of broad-spectrum antibiotics on the normal flora. Compare and contrast between the beneficial and the harmful effects of the normal flora .

Microbes and human relationship There are at least 3 types of relationships based on the quality of the relationship for each member of the symbiotic association: Mutualism Commensalism Parasitism

Mutualism Both members of the association benefit. One classic mutualistic association is that of the the lactic acid bacteria that live on the vaginal epithelium of a woman. The bacteria are provided habitat with a constant temperature and supply of nutrients (glycogen) in exchange for the production of lactic acid, which protects the vagina from colonization and disease caused by yeast and other potentially harmful microbes. Lactobacilli in association with a vaginal epithelial cell (CDC)

Commensalism One organism benefits but the other is neutral (there is no harm or benefit). Staphylococcus epidermidis, a consistent inhabitant of the skin of humans. The bacterium produces lactic acid that protects the skin from colonization by harmful microbes that are less acid tolerant. But it has been suggested that other metabolites that are produced by the bacteria are an important cause of body odors (good or bad, depending on your personal point of view) and possibly associated with certain skin cancers. "Commensalism" best works when the relationship between two organisms is unknown and not obvious. Staphylococcus epidermidis (CDC)

Parasitism One organism benefits at the cost of the other. A parasite that causes disease in its host is called a pathogen. Some parasitic bacteria live as normal flora of humans while waiting for an opportunity to cause disease. Other nonindigenous parasites generally always cause disease if they associate with a nonimmune host.

Normal flora “Normal microbial flora” –population of microorganisms that inhabit the skin and mucous membranes of healthy normal persons. Definition: The microorganisms that grow in or on the body without producing obvious harmful effects on the host makes up the normal flora. Symbiotic relationships with the host. The normal flora is acquired at birth.

The skin and mucous membranes always harbor a variety of microorganisms that can be arranged into two groups: (1)The resident flora fixed types of microorganisms regularly found in a given area at a given age if disturbed, it promptly reestablishes itself.

(2) The transient flora Nonpathogenic or potentially pathogenic microorganisms Inhabit the skin or mucous membranes for hours, days, or weeks It is derived from the environment Does not establish itself permanently on the surface. Members of the transient flora are of little importance so long as the normal resident flora remains intact. However, if the resident flora is disturbed, transient microorganisms may colonize, proliferate and produce disease.

Factors that determine normal flora Their flourishing in a given area depends upon many physiologic factors : Temperature: highly constant temperature (warm-blood animals). Moisture Presence of certain nutrients: rich in organic nutrients and growth factors Inhibitory substances Relatively constant conditions of pH and osmotic pressure

Useful effects of the normal microbial biota on the human body The normal flora synthesize and excrete vitamins prevent colonization by pathogens may antagonize other bacteria stimulate the development of certain tissues, eg:Peyer's patches in the GI tract. stimulate the production of natural antibodies.

Useful as well as the harmful effects of the normal microbial biota on the human body The resident flora of certain areas plays a definite role in maintaining health and normal function. Members of the resident flora in the intestinal tract synthesize vitamin K and aid in the absorption of nutrients. On mucous membranes and skin, the resident flora may prevent colonization by pathogens and possible disease through “bacterial interference”.

It may involve competition for receptors or binding sites on host cells, competition for nutrients, mutual inhibition by metabolic or toxic products, mutual inhibition by antibiotic materials or other mechanisms. Suppression of the normal flora -organisms from the environment or from other parts of the body-behave as opportunists and may become pathogens. Members of the normal flora may themselves produce disease under certain circumstances.

These organisms are adapted to the noninvasive mode of life defined by the limitations of the environment. If forcefully removed from the restrictions of that environment and introduced into the bloodstream or tissues, these organisms may become pathogenic. For example, streptococci of the viridans group are the most common resident organisms of the upper respiratory tract.

If large numbers of them are introduced into the bloodstream (e. g If large numbers of them are introduced into the bloodstream (e.g., following tooth extraction or tonsillectomy), they may settle on deformed or prosthetic heart valves and produce infective endocarditis. Small numbers occur transiently in the bloodstream with minor trauma (e.g., dental scaling or vigorous brushing).

Bacteroides are the commonest resident bacteria of the large intestine and are quite harmless in that location. If introduced into the free peritoneal cavity or into pelvic tissues along with other bacteria as a result of trauma, they cause bacteremia.

Specific adherence Most bacteria can colonize a specific tissue or site because they can adhere to that tissue or site in a specific manner that involves complementary chemical interactions between the two surfaces. Specific adherence involves biochemical interactions between bacterial surface components (ligands or adhesins) and host cell molecular receptors. The bacterial components that provide adhesins are molecular parts of their capsules, fimbriae, or cell walls. The receptors on human cells or tissues are usually glycoprotein molecules located on the hostcell or tissue surface.

Various streptococci in a biofilm in the oral cavity.

Normal flora of the vagina Soon after birth, aerobic lactobacilli appear in the vagina and persist as long as the pH remains acid (several weeks). When the pH becomes neutral (remaining so until puberty), a mixed flora of cocci and bacilli is present. At puberty, aerobic and anaerobic lactobacilli reappear in large numbers and contribute to the maintenance of acid pH through the production of acid from carbohydrates, particularly glycogen.

This appears to be an important mechanism in preventing the establishment of other, possibly harmful microorganisms in the vagina. If lactobacilli are suppressed by the administration of antimicrobial drugs, yeast or various bacteria increase in the numbers and cause irritation and inflammation After menopause, lactobacilli again diminish in number and a mixed flora returns.

The normal vaginal flora often includes also group B hemolytic Streptococci, anaerobic Streptococci (peptostreptococci), Prevotella species, Clostridia, Gardnerella vaginalis, Ureaplasma urealyticum, and sometimes Listeria or Mobiluncus species. The cervical mucus has antibacterial activity and contains lysozyme. Vaginal organisms present at time of delivery may infect the newborn (eg. group B streptococci).

Lactobacilli in the oral cavity probably contribute to acid formation that leads to dental caries.  Lactobacillus acidophilus colonizes the vaginal epithelium during child-bearing years and establishes the low pH that inhibits the growth of pathogens.

Gram stain of a species of Micrococcus, commonly isolated from the skin and nasal membranes of humans.

Streptococcus pyogenes Streptococcus pyogenes - Group A, Beta-hemolytic streptococci. Cause tonsillitis (strep throat), pneumonia, endocarditis. Some streptococcal diseases can lead to rheumatic fever or nephritis which can damage the heart and kidney.

Neisseria meningitidis Neisseria and other Gram-negative cocci - frequent inhabitants of the upper respiratory tract, mainly the pharynx. Neisseria meningitidis - important cause of bacterial meningitis.

E. coli E. coli - a consistent resident of the small intestine. Klebsiella, Enterobacter and Citrobacter also seen Some strains of E. coli are pathogens that cause intestinal infections, urinary tract infections and neonatal meningitis.

Pseudomonas aeruginosa The commonest opportunistic pathogen of humans that can invade virtually any tissue.  Leading cause of hospital-acquired (nosocomial) Gram-negative infections, but its source is often exogenous (from outside the host).

Effects of broad-spectrum antibiotics on the normal flora Normal flora (normal microbiota) (a) the drug destroy the friendly/helpful bacteria (normal microbial flora) (b) Because they have such a wide spectrum of activity, [tetracyclines] destroy the normal intestinal microflora and often produce severe gastrointestinal disorders Superinfection (a) Knocking out these non-pathogenic bacteria can lead to disease (e.g., diarrhea, Clostridium difficile -associated colitis, Candida vaginal yeast infections, etc.)

(b) Normal flora can compete with pathogenic bacteria (microbial antagonism), thus preventing disease; removing these flora can thus make an individual more susceptible to subsequent disease (c) The replacement of a normal flora member by a pathogen is called superinfection