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Human immune system

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Laboratory Diagnosis, Prevention and Treatment
of Bacterial Infection

Manifestations of Infection: Signs and symptoms vary according to the site and severity of infection. Diagnosis requires a composite of information, including history, physical examination, radiographic findings, and laboratory data.

Microbial Causes of Infection: Infections may be caused by bacteria, viruses, fungi, and parasites. The pathogen may be exogenous (acquired from environmental or animal sources or from other persons) or endogenous (from the normal flora).

Specimen Selection, Collection, and Processing

The quantity material must be adequate

Specimens are selected on the basis of signs and symptoms, should be representative of the disease process

Contamination of the specimen must be avoided by using only sterile equipment and aseptic precautions

The specimen must be taken to the laboratory and examined promptly. Special transport media may be helpful.

Meaningful specimens to diagnose bacterial infections must be secured before antimicrobial drugs are administered.

Microbiologic Examination

Culture:Isolation of infectious agents frequently requires specialized media. Nonselective (noninhibitory) media permit the growth of many microorganisms. Selective media contain inhibitory substances that permit the isolation of specific types of microorganisms.

Microbial Identification: Colony and cellular morphology may permit preliminary identification. Growth characteristics under various conditions, utilization of carbohydrates and other substrates, enzymatic activity, immunoassays, and genetic probes are also used.

Antimicrobial Susceptibility: Microorganisms, particularly bacteria, are tested in vitro to determine whether they are susceptible to antimicrobial agents.

Serodiagnosis:A high or rising titer of specific IgG antibodies or the presence of specific IgM antibodies may suggest or confirm a diagnosis.

Direct Examination and Techniques: Direct examination of specimens reveals gross pathology. Microscopy may identify microorganisms. Immunofluorescence, immuno-peroxidase staining, and other immunoassays may detect specific microbial antigens. Genetic probes identify genus- or species-specific DNA or RNA sequences.

Artificial active immunity

Toxoids: a modified form of the toxin that preserves its antigenicity but has lost its toxicity. This has been spectacularly successful with tetanus and diphtheria.

Inactivated vaccines: The production of protective antibodies is stimulated by using the killed (inactivated) organisms This is done as a routine with vaccines against pertussis (whooping cough) , typhoid and influenza. There is also an inactivated polio vaccine.

Attenuated live vaccines : The approach is to use suspensions of living organisms that are reduced in their virulence (attenuated) but still immunogenic. This strategy has yielded: BCG, mumps, measles , and rubella vaccines (now combined); the live virus polio vaccine.

Special vaccines: polysaccharide vaccine, subunit vaccine, ( conjugate vaccine, bio-engineered vaccine, chemical vaccine, synthetic vaccine ), nucleic acid vaccine, idiotype vaccine, autovaccine, etc.

Artificial passive immunity

Antitoxin: e.g. Tetanus antitoxin and diphtheria antitoxin. It is raised in the horse .It is most important to give an intented recipient of equine serum a prior test dose to exclude hypersensitivity subjects who may have been sensitized by a previous dose of equine serum.

Pooled immunoglobulin: It contains the normal repertoire of antibodies for an adult, and can protect against hepatitis A, and measles.

Specific immunoglobulin: Preparations of specific immunoglobulin are available for passive immunization against tetanus, hepatitis B, rabies, varicella-zoster.


Active-passive immunity

involves giving both a vaccine to provide long-term protection (preventive infection) and immune globulin to provide immediate protection (therapeutic and preventive infectious disease).


DI = duration of illness, DH = duration of hospitalization, B/BF = blood/body fluid precautions, D/S = drainage/secretion precautions, E = enteric precautions, C = contact isolation, S = strict isolation, R = respiratory isolation, TB = tuberculosis isolation, U = universal precautions.

General procedure for collecting and processing specimens for aerobic and/or anaerobic bacterial culture

Agglutination test in which inert particles (latex beads or heat-killed S aureus Cowan 1 strain with protein A) are coated with antibody to any of a variety of antigens and then used to detect the antigen in specimens or in isolated bacteria.


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