Microbiological Classification Of Infectious Diseases

Microbiological Classification ofInfectious Diseases Disease is a disturbance in the state of health Microbes cause disease in the course of stealingspace, nutrients, and/or living tissue from theirsymbiotic 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 CHARACTERISTICSOF INFECTIOUS AGENTS Infectivity – the ability to infect a host Pathogenicity – the ability to causedisease in the host Virulence – the ability to cause severedisease in the host Immmunogenicity –the ability to inducean immune response in the host

Infectious Disease TermsInfectious dose – number of organisms needed to successfully infectLatent period - exposure to infectiousness intervalIncubation period – interval from exposure to clinical symptomsInfectious period – interval during which host can transmit infectionReproductive rate – ability of an agent to spread in break – limited spreadEndemic – usually present; steady prevalenceEpidemic – rapid spreadPandemic – occurring across countries and in multiple populations

Important Terms

MODES OF TRANSMISSION Direct– Droplet– Aerosol– Skin to skin Indirect– Fomites (clothes, blankets, door handles etc)– Vectors (e.g. mosquitoes)– Food and water– Intermediate hosts (e.g. snails)

CLINICAL RESPONSES TOINFECTION BY AN AGENT Inapparent infection – no clinical symptomsgenerated Carrier state – usually no clinical symptomsbut host can transmit infection for longperiods Clinical symptoms––––Mild diseaseSevere diseaseResidual impairmentdeath

CLASSIFICATION OFINFECTIOUS AGENTS (1 of 2) Bacteria – survive on appropriate media, staingram-positive or -negative Viruses – obbligate intracellular parasites whichonly replicate intracellularly (DNA, RNA) Fungi – non-motile filamentous, branchingstrands of connected cells Metazoa – multicellular animals (e.g.parasites)with complicated life cycles often involvingseveral hosts

CLASSIFICATION OFINFECTIOUS AGENTS (2 of 2) Protozoa – single cell organisms with a welldefined nucleus Rickettsia – very small bacteria spread by ticks Prions – unique proteins lacking genetic molecules Chlamydia – bacteria lacking cell walls

Size Comparison of Microbes

Bacterial Cell StructureJames D. Dick, PhD, Johns Hopkins University

Taxonomy of BacteriaStrainO157:H7

Microbiological Classification ofInfectious Diseases Bacteria are classified by their Gram staincharacteristics. Gram staining is the application of acrystal violet dye to a culture of bacteria.Bacteria that retain the color of the dye arecalled Gram positive; bacteria that don'tare Gram negative.– The Gram stain attaches to peptidoglycan inthe bacterial cell wall. In Gram-negative bacteria, the peptidoglycan layeris protected by an outer membrane.

Microbiological Classification ofInfectious Diseases

Microbiological Classification ofInfectious Diseases

Microbiological Classification ofInfectious Diseases Viruses are acellular, obligate intracellularorganisms. 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 theypossess a lipoprotein coat that surrounds the capsidand is acquired from infected host cell membrane.– Viruses that lack an envelope are “naked.”

Microbiological Classification ofInfectious Diseases Viruses are typically classified by:– Genetic material (DNA vs. RNA)– Strandedness (single vs. double)– Size and shape of the capsid and whether itsenveloped or non-enveloped– Method of replication

Microbiological Classification ofInfectious Diseases

Microbiological Classification ofInfectious Diseases

Microbiological Classification ofInfectious Diseases

VirusesEntry into the Host Cell with an Envelope

VirusesEntry into the Host Cell with an Envelope

Microbiological Classification ofInfectious Diseases All fungi are chemoheterotrophs Pathogenic fungi have two forms: yeasts(unicellular) and molds (multicellular) Some fungi are dimorphic (this isparticularly true for the pathogenic fungi) Molds grow as filamentous, branchingstrands of connected cells known ashyphae

Microbiological Classification ofInfectious Diseases Types of Parasites:– Protozoa: Single-celled, microscopic organisms thatcan perform all necessary functions of metabolismand reproduction. Some protozoa are free-living,while others parasitize other organisms for theirnutrients and life cycle. The morphology of protozoa varies widely and includes oval,spherical and elongated cells that can range in size from 510 to 1-2 mm. Structurally, the protozoa resemble other eukaryotic cells andpossess a cytoplasmic membrane that encloses cytoplasmcontaining membrane-bound nuclei, mitochondria, 80sribosomes and a variety of organelles.

Microbiological Classification ofInfectious Diseases Types of Parasites:– Helminths: A large, multicellular organism (worm) thatis generally visible to the naked eye in its adultstages.– Helminths can be free-living or parasitic. Nematodes: Roundworms Trematodes: Flukes Cestodes: Tapeworms

Microbiological Classification ofInfectious Diseases Prions are abnormal, transmissible agentsthat are able to induce abnormal folding ofnormal cellular prion proteins in the brain,leading to brain damage and thecharacteristics signs and symptoms of thedisease. Prion diseases are usually rapidlyprogressive and always fatal.

– 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