MEDICAL MICROBIOLOGY

LECTURE 1. OVERVIEW OF MEDICAL MICROBIOLOGY. CLASSIFICATION OF INFECTIOUS AGENTS

Omirbekova Anel A., PhD, Associated Professor

Medical Microbiology is a branch of medical and biological sciences that deals with the pathogens causing disease (1), the ways they produce disease (2), diagnosis (3), treatment (4), host response –immunity (5) and prevention (6).

Medical microbiologist may:

• study the characteristics of pathogens, their ways of transmission, mechanisms of infection and growth;

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• identify pathogens and suggest treatment options to physician;
• identify potential health risks to the community or monitore the evolution of potentially virulent or resistant strains of microbes;
• may assist in preventing or controlling epidemics and outbreaks of disease;
• study common, non-pathogenic species to determine whether their properties can be used to develop antibiotics or other treatment methods.

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While epidemiology is the study of the patterns, causes, and effects of health and disease conditions in populations, medical microbiology primarily focuses on the presence and growth of microbial infections in individuals, their effects on the human body and the methods of treating those infections.

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A pathogen is an organism capable of invading the body and causing disease. Such an organism is termed pathogenic.

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An infectious disease is an illness caused by a pathogen,

which invades body tissues and causes damage.

Pathogenesis

Pathogens can be classified into five main groups:

• viruses
• bacteria
• fungi
• protozoa
• and one type of infectious protein called prion

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Branches of Medical Microbiology

• Bacteriology

• Parasitology

• Mycology

• Virology

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Microbiological Classification of Infectious Diseases

• Disease is a disturbance in the state of health
• Microbes cause disease in the course of stealing space, nutrients, and/or living tissue from their symbiotic hosts (e.g., us)
• To do this, microbes do most of the following:
• Gain access to the host (contamination)
• Adhere to the host (adherence)
• Replicate on the host (colonization)
• Invade tissues (invasion)
• Produce toxins or other agents that cause host harm (damage)

BIOLOGIC CHARACTERISTICS OF INFECTIOUS AGENTS

Infectivity – the ability to infect a host

Pathogenicity – the ability to cause disease in the host

Virulence – the ability to cause severe disease in the host

Immmunogenicity –the ability to induce an immune response in the host

INFECTIOUS DISEASE TERMS

Infectious dose – number of organisms needed to successfully infect

Latent period - exposure to infectiousness interval

Incubation period – interval from exposure to clinical symptoms

Infectious period – interval during which host can transmit infection

Reproductive rate – ability of an agent to spread in populations Endemic – usually present; steady prevalence

Epidemic – rapid spread

Pandemic – occurring across countries and in multiple populations

Important Terms

CLASSIFICATION OF INFECTIOUS AGENTS

Bacteria – survive on appropriate media, stain gram-positive or -negative

Viruses – obbligate intracellular parasites which only replicate intracellularly (DNA, RNA)

Fungi – non-motile filamentous, branching strands of connected cells

Metazoa – multicellular animals (e.g.parasites) with complicated life cycles often involving several hosts

Protozoa – single cell organisms with a well- defined nucleus

Prions – unique proteins lacking genetic molecules

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Size Comparison of Microbes

Taxonomy of Bacteria

Strain

O157:H7

Microbiological Classification of Infectious Diseases

Bacteria are classified by their Gram stain characteristics.

Gram staining is the application of a crystal violet dye to a culture of bacteria. Bacteria that retain the color of the dye are called Gram positive; bacteria that don't are Gram negative.

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Microbiological Classification of Infectious Diseases

Bacteria are classified by their Gram stain characteristics.

Gram staining is the application of a crystal violet dye to a culture of bacteria. Bacteria that retain the color of the dye are called Gram positive; bacteria that don't are Gram negative.

• The Gram stain attaches to peptidoglycan in the bacterial cell wall.
• In Gram-negative bacteria, the peptidoglycan layer is protected by an outer membrane.

Microbiological Classification of Infectious Diseases

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Microbiological Classification of Infectious Diseases

• Viruses are acellular, obligate intracellular organisms.
• The complete infectious virus is termed a virion.
• The virion consists of the specific nucleic acid (DNA or RNA) surrounded by a protein coat (capsid).
• Some viruses are enveloped which means that they possess a lipoprotein coat that surrounds the capsid and is acquired from infected host cell membrane.
• Viruses that lack an envelope are “naked.”

Microbiological Classification of Infectious Diseases

• Viruses are typically classified by:
• Genetic material (DNA vs. RNA)
• Strandedness (single vs. double)
• Size and shape of the capsid and whether its enveloped or non-enveloped
• Method of replication

Microbiological Classification of Infectious Diseases

Microbiological Classification of Infectious Diseases

Microbiological Classification of Infectious Diseases

Kingdom Fungi

• 100,000 species divided into 2 groups:
• macroscopic fungi (mushrooms, puffballs, gill fungi)
• microscopic fungi (molds, yeasts)
• Majority are unicellular or colonial; a few have cellular specialization

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Microscopic Fungi

• Exist in two morphologies:
• yeast – round ovoid shape, asexual reproduction
• hyphae – long filamentous fungi or molds
• Some exist in either form – dimorphic – characteristic of some pathogenic molds

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Fungal Nutrition

• All are heterotrophic
• Majority are harmless saprobes living off dead plants and animals
• Some are parasites, living on the tissues of other organisms, but none are obligate; mycoses – fungal infections
• Growth temperature 20^o-40^oC
• Extremely widespread distribution in many habitats

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Fungal Organization

• Most grow in loose associations or colonies
• Yeast – soft, uniform texture and appearance
• Filamentous fungi – mass of hyphae called mycelium; cottony, hairy, or velvety texture
• hyphae may be divided by cross walls – septate
• vegetative hyphae – digest and absorb nutrients
• reproductive hyphae – produce spores for reproduction

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Fungal Reproduction

• Primarily through spores formed on reproductive hyphae
• Asexual reproduction – spores are formed through budding or mitosis; conidia or sporangiospores

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Fungal Reproduction

• Sexual reproduction – spores are formed following fusion of male and female strains and formation of sexual structure
• Sexual spores and spore-forming structures are one basis for classification.

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Fungal Classification

Subkingdom Amastigomycota – terrestrial

inhabitants including those of medical importance:

1. Zygomycota – zygospores; sporangiospores and some conidia
2. Ascomycota – ascospores; conidia
3. Basidiomycota – basidiospores; conidia
4. Deuteromycota – majority are yeasts and molds; no sexual spores known; conidia

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Fungal Identification

• Isolation on specific media
• Macroscopic and microscopic observation of:
• asexual spore-forming structures and spores
• hyphal type
• colony texture and pigmentation
• physiological characteristics
• genetic makeup

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Roles of Fungi

• Adverse impact
• mycoses, allergies, toxin production
• destruction of crops and food storages
• Beneficial impact
• decomposers of dead plants and animals
• sources of antibiotics, alcohol, organic acids, vitamins
• used in making foods and in genetic studies

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Protozoa

• 65,000 species
• Vary in shape, lack a cell wall
• Most are unicellular; colonies are rare
• Most are harmless, free-living in a moist habitat
• Some are animal parasites and can be spread by insect vectors.
• All are heterotrophic.
• Feed by engulfing other microbes and organic matter

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Protozoa

• Most have locomotor structures – flagella, cilia, or pseudopods.
• Exist as trophozoite - motile feeding stage
• Many can enter into a dormant resting stage when conditions are unfavorable for growth and feeding – cyst.
• All reproduce asexually, mitosis or multiple fission; many also reproduce sexually – conjugation.

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Protozoan Identification

• Classification is difficult because of diversity.
• Simple grouping is based on method of motility, reproduction, and life cycle.
1. Mastigophora – primarily flagellar motility, some flagellar and amoeboid; sexual reproduction; cyst and trophozoite
2. Sarcodina – primarily ameba; asexual by fission; most are free-living
3. Ciliophora – cilia; trophozoites and cysts; most are free-living, harmless
4. Apicomplexa – motility is absent except male gametes; sexual and asexual reproduction; complex life cycle – all parasitic

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Important Protozoan Pathogens

• Pathogenic flagellates
• Trypanosomes – Trypanosoma
• T. brucei – African sleeping sickness
• T. cruzi – Chaga’s disease; South America
• Infective amebas
• Entameba histolytica – amebic dysentery; worldwide

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Parasitic Helminths

• Multicellular animals, organs for reproduction, digestion, movement, protection
• Parasitize host tissues
• Have mouthparts for attachment to or digestion of host tissues
• Most have well-developed sex organs that produce eggs and sperm.
• Fertilized eggs go through larval period in or out of host body.

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Major Groups of Parasitic Helminths

1. Flatworms – flat, no definite body cavity; digestive tract a blind pouch; simple excretory and nervous systems
• cestodes (tapeworms)
• Trematodes or flukes, are flattened , nonsegmented worms with sucking mouthparts.
2. Roundworms (nematodes)- round, a complete digestive tract, a protective surface cuticle, spines and hooks on mouth; excretory and nervous systems poorly developed

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Helminths

• 50 species parasitize humans.
• Acquired though ingestion of larvae or eggs in food; from soil or water; some are carried by insect vectors
• Afflict billions of humans

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Helminth Classification and Identification

• Classify according to shape, size, organ development, presence of hooks, suckers, or other special structures, mode of reproduction, hosts, and appearance of eggs and larvae
• Identify by microscopic detection of adult worm, larvae, or eggs

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