Aspects of Medical Microbiology

Prokaryotic and Eukaryotic Microorganisms:

As it has been pescribed by Woese that has been gaining general acceptance in recent years, the world of living beings has been classified into three classes called domains. This domains include: bacteria, archaea, and eucarya. In this system,every and each domain has been further further classified into groups below:

• Human Pathogens
• Subcellular biological entities and
• Prokaryotic microorganisms and also
• Eukaryoties microorganisms
• Animals
• Prions ieinfection proteins
• Chlamydiae length 0.3–1 lm
• Fungi ,with yeasts 5–10 lm, size of mold fungi Indeterminable
• Helminths or parasitic worms
• Viruses length20–200 nm
• Rickettsiae length 0.3–1 lm
• Protozoa length1–150 lm
• Arthropods
• Mycoplasmas
• Classic bacteria length 1–5 lm

Pathogen

kingdoms. Pathogenic microorganisms are found in the domains or groups of bacteria & Eucarya.

Bacteria, Archaea & Eucarya Bacteria. Thisese domains includes or posses the kingdom of the heterotrophic eubacteria and posseses all infectious human pathogen bacteria. The other kingdoms, for example that of the photosynthetic cyanobacteria, which are not pathogenic or and can be called as non infectous or are not of any danger..It is assumed and estimated that bacterial species on Earth surface including atmosphere number in the hundreds and thousands, of which only is about 5500 have been already discovered and described in detail with logic appropriarely..

Archaea. This domain includes forms and groups of microorganisms that live under extreme and vast environmental conditions,including thermophilic, hyperthermophilic, halophilic,the water lovers and methanogenic microorganisms. The earlier previous term for the archaea was archaebacteria (ancient bacteria), and they are indeed a kind of living importance as fossil. Thermophiles archaea thrive mainly in warm, moist biotopes like the hot springs at the top of so called as geothermal vents. The hyperthermophilics archaea, a more recent discovery, live around deep-sea volcanic plumes at temperatures exceeding abovew 1008C.

Eucarya. The domain includes all life forms with cells possessing a genuine nucleus. The plant and or animal kingdoms are well known as eukaryotic lifeforms. Pathogenic eukaryotic microorganisms includess usually the fungal and protozoan species.

Classic bacteria. These bacteria reproduce asexually by the process of binary transverse fission. They do not possess the nucleus that is also typical of Eucarya. The cell walls of these bacteria are rigid (with some exceptions, e.g., the mycoplasma).

Chlamydiae. Chlamydie are oblogate intracellular parasites that can reproduce in certain human cells only and are divided into two stages: the infectious, nonreproduccing particles called elementary bodies size 0.3 lm m and the noninfectious, intracytoplasmic, reproductive forms also called as initial (or reticulate) bodies size 1 lm.

Rickettsiae. These organisms are obligate intracellular parasites. they might be rodshaped as well as coccoid, that reproduce by transverse binary fission. The diameter of the individual cell is from 0.3–1 lm. size.

Mycoplasmas.These organisms are the bacteria without rigid cell walls. They are found in various types of forms, the most common being the coccoid cell (0.3–0.8 lm). A thread like structured organisms also exists in various lengths.

Fungi and Protozoa

Fungi. Fungi (Mycophyta) are nonmotile eukaryotes having rigid cell walls along with a classic cell nucleus. They contain no photosynthetic pigments and act as carbon heterotrophic, i.e, they make use of various organic nutrient substrates (in contrast to carbon autotrophic plants). Among more than 50 000 fungal species, only about 300 have been identified and detected human pathogens. Most fungal infections are caused due to the weakening of host immune defenses.

Protozoa. Protozoa are microorganisms in various sizes and forms could be free-living or parasitic. They have a nucleus containing in the chromosomes and organelles such as mitochondria may also be lacking in some cases.

Host–Pathogen Interactions

Many parasitic protozoa are transmitted by arthropods, where by multiplication and transformation or conversion into the infectious stage or agent take place in the vector. ie host.

Animals

Helminths. Parasitic worms are belonged to the animal kingdom. therefore these organisms are called as metazoan organisms with highly differentiated cell structures. Medically significant or important groups include the trematodes ie. lukes or flatworms cestodes (tapeworms), and nematodes (roundworms).

Arthropods. These group of animals are characterized by or possesses the characteristic an external chitin skeleton with segmented bodies, jointed legs,may be special mouthparts, and also many other specific features. Their feature as direct causative infectious agents of diseases is a minor one (mites, for example, cause scabies) as compared to their role as vectors that are transmitting viruses, bacteria, protozoa , and helminths.

Host–Pathogens Interactions

The factors determining or proving the genesis, clinical view and output of an infection include complex relationships between the host and invading organisms that differ very widely that depends on the pathogen involved. Despite this variability, a number of general principles apply to the interactions between the invading pathogen with its aggression factors and the host or animal with its defenses mechanism.. Since the pathogenesis and infecting abillity or infecting capability of bacterial infectious diseases has been researched very thoroughly, the following summary is based on the host–invader interactions seen in this type of infection.

The determinants of bacterial pathogenicity & virulence can be written below with the points as followed:

• Adhesion to host cells (adhesins).
• Breaching of anatomical barriers of the host, invasions, and colonization of tissues or aggressins.
• Strategies to overcome nonspecific defenses, the especially antiphagocytic process of mechanisms or impeding.
• Strategies to overcome and gain specific immunity, the most important of which is actually the production of IgA proteases or impeding, molecular mimicry and so-called immunogens.

• Damage to host tissues or cells due to direct bacterial cytotoxicity and exotoxins, and exoenzymes or aggressins.
• Damage due to inflammatory reactions in the physiology macroorganisms within an activation of complement and phagocytosis; induction of cytokine production (modulins). The above bacterial pathogenicity factors are confronted by the following host defense mechanisms:
• Nonspecific defenses including or possesing mechanical, humoral, and cellular systems. Phagocytosis is the most important process in this context.
• Specific immune responses that may be based on antibodies and specific reactions of T lymphocytes (see the chapter on immunology).
• The response of these defenses to infection thus involves the correlation of a number of different mechanisms. Defective defenses make it easier for an infection to take hold. Primary, innate defects or problems are rare, whereas acquired in lifetime, secondary immune defects or problems occur frequently, paving the way for infections by microorganisms known as “facultative pathogens” usually opportunists.

Basic Definitions or termsofInfectiology

The terms pathogenicity and virulence are not clearly and wisely defined in their relation to microorganisms. They are sometimes even used synonymously. It has been proposed and proved that pathogenicity can be be used to characterize a particular species identity and that virulence be used to describe the sum of the disease-causing properties and aligiblity of a population or strain of a pathogenic species Pathogenicity and virulence in the microorganism correspond to susceptibility in a host species and disposition in a specific host organism, whereby an individual may be anywhere from highly disposed to resistant.

Determinants of Bacterial Pathogenicity and Virulence

Relatively little is known about the factors determining and conforming the pathogenicity and virulence of microorganisms, and most of what we do know concerns the disease-causing mechanisms of bacteria.

Basic Infectious Terminology I (Pathogen)

Term Explanation

• Saprophytes These microorganisms are nonpathogenic; their natural habitat is dead organic matter decay matter .
• Parasites Unicellular or metazoan organism living in and on an organism of another species or host on the expenseof the host
• Commensals Normal inhabitants of skin and mucosa; the normal flora is thus the total commensal population

• Pathogenic microorganisms: Classic disease-causing pathogensOpportunists or facultatively pathogenic .Microorganisms Can cause disease in immune compromised individuals given an “opportune” situation; these arefrequently germs of the normal flora or occasionally from the surrounding environment, animals, or other germ carriers
• Pathogenicity Capacity of a pathogen species to cause disease Virulence Sum of the disease-causing properties of a strainof a pathogenic species
• Incubation period Time between infection and manifestation and appearence of disease symptoms; this specific disease characteristic can also be measured in hours, days, weeks,or even years
• Prepatency: A parasitological term: time between infectionand first appearance of products of sexual reproductionof the pathogen (e.g., worm eggs in stool of a host with helminthosis)
• Infection spectrum: The totality of host species “susceptible” to infection by a given pathogen
• Minimum infective dose: Smallest number of pathogens sufficient to cause an infection
• Mode of infection :Method or pathway used by pathogen to invade host

References:

D greenwood, Slack RCB and J Peutherer. Medical microbiology. 2001.

JG College, AG Fraser and BP Marmion. Practical Medical microbiology. Fourteen Edition. Churchill Livingstone, 1996.

JP Micheal, ECS Chan and NR Krieg. Microbiology. Fifth Edition. Delhi: Mcgraw-hill, 1993.

M Cheesbrugh. Medical laboratory manual for tropical countries. London, 2007.