6.1 Bacteria and disease

pathogens

disease causing microorganisms

In a culture

microorganisms are provided with the nutrients, level of oxygen, pH and temperature that they require to grow large numbers so they can be observed and measured

sterile

something that is free from living microorganisms and their spores

Nutrient medium

substance used for the culture of microorganisms, which can be in liquid form (nutrient broth) or in solid form (usually nutrient agar)

Nutrient broth

liquid nutrient for culturing microorganisms, commonly used in flasks, test tubes or bottles

Nutrient agar

jelly extracted from seaweed and used as a solid nutrient for culturing microorganisms, commonly used in Petri dishes

selective medium

a growth medium for microorganisms containing a very specific mixture of nutrients, so only a particular type of microorganism will grow on it

inoculation

the process by which microorganisms are transferred into a culture medium under sterile conditions

haemocytometer

a thick microscope slide with a rectangular identation and etched grid lines that is used to count cells

turbidimetry

a method of measuring the concentration of a substance by measuring the amount of light passing through it

turbid

something that is opaque, or thick with suspended matter

total viable cell count

a measure of the number of cells that are alive in a given volume of a culture

generation time

the time span between bacterial divisions

lag phase

when bacteria are adapting to their new environment and are not yet reproducing at their maximum rate

Log phase

when the rate of bacterial reproduction is close to or at its theoretical maximum, repeatedly doubling in a given time period

Stationary phase

when the total growth rate is zero as the number of new cells formed by binary fission is equalled by the numbers of cells dying due to factors including competition for nutrients, lack of essential nutrients, an accumulation of toxic waste products and possibly lack of, or competition for, oxygen.

death phase

when reproduction has almost ceased and the death rate of cells is increasing so that the population number falls

endotoxins

are lipopolysaccharides that are an integral part of the outer layer of the cell wall of Gram-negative bacteria

Exotoxins

soluble proteins that are produced and released into the body by bacteria as they metabolise and reproduce in the cells of their host

Lipopolysaccharides

large molecules containing a lipid element and a polysaccharide element (found in the cell walls of Gram-negative bacteria)

Tuberculosis

a lung disease cause by Mycobacterium tuberculosis and M. bovis.

primary infection

the initial stage of tuberculosis when M. tuberculosis has been inhaled into the lungs, invaded the cells of the lungs and multiplied slowly, often causing no obvious symptoms.

tubercule

the result of a healthy immune response to an infection by M. tuberculosis. A localised inflammatory response forms a mass of tissue containing dead bacteria and macrophages.

Antibiotic

a drug that either destroys microorganisms or prevents them from growing and reproducing

Selective toxicity

a substance is toxic against some types of cells or organisms but not others

Penicillin

the first antibiotic discovered. It affects the formation of bacterial cell walls, and it is bactericidal

Bacteriostatic

antibiotics inhibit the growth of bacteria

Tetracycline

a bacteriostatic antibiotic that inhibits proteins synthesis

Bactericidal

bactericidal antibiotics kill bacteria

Antibiotic-resistant microorganism

not affected by an antibiotic, even one that may have been effective in the past

Methicillin-resistant Staphylococcus aureus (MRSA)

a strain of S. aureus that is resistant to several antibiotics, including methicillin

Clostridium difficile

a type of bacteria that often exists in the intestines and causes no problems unless it becomes dominant as a result of the normal gut flora being removed/damaged by antibiotic treatment.

Healthcare-associated infections (HCAIs)

infections that are acquired by patients whilst they are in hospitals or care facilities. They may be the result of poor hygiene between patients, or antibiotic treatment, and may be antibiotic resistant.

dilution plating

a method used to obtain a culture plate with a countable number of bacterial colonies