General Microbiology

This chapter deals with the basic mechanisms of microbiology and epidemiology of infectious diseases. There are endogenous infections caused by physiologic bacteria or reactivated inections due to immunologic weakness, and exogenous infections caused by agents that enter the organism.

Furthermore, we look at local and generalized infections. The latter have three stages: incubation period, generalization, manifestation. Whether an agent causes an infection depends on pathogenity and virulence factors, e.g. adherence, capsules, endotoxins, exotoxins. The body has several mechanisms for fighting infections: unspecific local resistance by skin and mucous membranes, unspecific systemic resistance (temperature, inflammation, phagocytosis, complement, interferon, CRP), and specific resistance, also called immunity, either natural or acquired.

Definitions

An infection is the invasion, settlement and proliferation of microorganisms (bacteria, viruses, fungi, parasites) in a host organism. The damaging effect of the agent and the body's reaction manifest in clinical symptoms and is called infectious disease. We distinguish bacterial infection from bacterial intoxication caused by bacterial toxins.

The incubation period is the time period between invasion and the manifestation of symptoms.

Stages

An infection may progress as follows: Latent infection, Subclinical infection, Clinically manifest infection.

Sources

Exogenous. Endogenous, e.g. catheter (staph. epidermidis), canker (cand. albicans), Endocarditis lenta (staph. vir., staph. epid. or enterococc.), urinary tract (e. coli), cholecystitis, appendicits (e. coli), pneumonia (streptoc. pneumoniae or haemophilus influenzae; reactivated tuberculosis, herpes labialis or genitalis.

Spread

Local: Mostly bacteria, fungi and parasites. No incubation period. Generalized: During incubation period, bacteria multiply in regional lymph nodes. After spreading by blood or lymph vessels, organ symptoms occur, often leaving immunity after the disease. Sepsis is the constant or periodic inundation of agents (not viruses) into the blood stream. There is no periodic general reaction and no spontaneous recovery. Symptoms of sepsis are: bacteriemia, intermittent temperature with chills, spleen tumor, blood count changes, shock. Possibly septic infarctions and metastatic abscesses. Bacteriemia shows agents in blood without organ manifestations. Always occurs with cyclic general infection.

Positive Identification

The agent should be ascertained. The agent should be isolated from the affected organ tissue and cultured in vitro. Infecting a test animal should cause typical symptoms. The agent should be isolated from the sick test animal. (Henle-Koch scheme)

Pathogenicity and Virulence

Pathogenicity is the ability to cause disease. Virulence is the degree of pathogenicity, depending on the host immunity and the agent's infectiousness, its tissue affinity and toxicity. Persistence means that the mostly intracellular agents cause no symptoms and cannot be reached by resistance mechanisms. They may cause a relapse later.

Pathogenicity factors are: adhesion, capsules, toxins (endotoxins; exotoxins, e.g. enterotoxins, neurotoxins); exoenzymes (collagenase, hyluronidase, streptokinase, coagulae, hemolysines, leukocidines).