Microbio Lab 8 Flashcards
antimicrobials
any compounds that kill or inhibit microorganisms
antibiotics
antimicrobials, usually of low molecular weight, naturally produced by microorganisms to inhibit or kill other microorganisms
ex. penicillin and streptomycin
semi-synthetics
antimicrobials that are chemically synthesized in the laboratory and are not produced by microbial biosynthesis at all
ex. sulfa drugs
healthcare acquired infections (HAIs)
formerly nosocomial infections; acquired in hospitals; MRSA is responsible for many hospital infections
Kirby-Bauer method
used to determine the sensitivity or resistance of a bacterium to an antimicrobial
performed by streaking a standardized inoculum of the test organism on Mueller-Hinton medium, and then paper disks containing specific concentrations of an antimicrobial are deposited on the agar surface. The antimicrobial diffuses out from the disk into the agar, forming a concentration gradient. If the agent inhibits or kills the test organism, there will be a zone around the disk where no growth occurs, called the zone of inhibition.
Mueller-Hinton II agar is the recommended medium with pH between 7.2 and 7.4
zone of inhibition
the zone around the disk where no growth occurs if an agents inhibits the microorganism; affected by diffusibility of the agent, size of the inoculum, the type of medium, and other factors
antiseptics
substances such as alcohol or betadine that inhibit microbial growth or kill microorganisms but they are gentle enough to be applied to living tissue; do not destroy endospores
disinfectants
chemical agents that are applied to inanimate objects such as floors, walls, and tabletops to kill microorganisms; damaging to living tissue
sterilants; sporocides
destroy all microbial life, including endospores
ex. ethylene oxide
sanitizers
agents that reduce microbial numbers to a safe level but do not completely eliminate all microbes
bacteriostatic
if a particular agent only inhibits the growth of bacterial cells but does not kill them
bacteriocidal
agents that kill bacterial cells
Heat affects…
proteins (enzymes)
nucleic acids (DNA and RNA structure)
other small molecules
thermal death point
the lowest temperature at which a population of the target organism is killed in 10 minutes
thermal death time
the shortest time required to kill a suspension of cells or spores under defined conditions at a given temperature
UV radiation at 260 nm
the most germicidal because it the wavelength at which DNA maximally absorbs UV light
pyrimidine dimers
covalent bond between two adjacent thymine or cytosine molecules in a DNA strand; caused by absorption of UV light
SOS system
enzymatically removes the dimers and inserts in their place new pyrimidine molecules.
Ultraviolet (UV) Light
Nonionizing short-wavelength radiation that falls between 4nm and 400nm in the visible spectrum; more germicidal than either visible light or infrared radiation
Why are endospores resistant to to UV light?
1) the DNA of endospores is protected by small acid-soluble proteins that bind to DNA and alter its conformation, thereby protecting it from photochemical damage
2) a unique spore photo-product is generated by UV light in endospores that functions in the enzymatic repair of damaged DNA during endospore germination
Antibiotic resistant strains
Staphylococcus aureus Pseudomonas aeruginosa Mycobacterium tuberculosis Neisseria gonorrhoeae Enterococcus
They are a problem in HAIs and is a major health problem
Narrow-spectrum antimicrobials
Specific in target
Ex. Targets gram-positive or gram-negative
Broad-spectrum antimicrobials
Effective against both gram- positive and negative organisms
Types of modes of action for antimicrobials
Target cell wall synthesis (penicillin)
Target DNA and RNA synthesis (cipro, rifampin)
Target protein synthesis (tetracyclines, streptomycin)
Target vitamin synthesis (sulfa drugs)
Affects whether an antimicrobial is narrow or broad
Permeability also affects function (harder to penetrate gram-negative cells)
Differentiate between the following and provide one example of each.
A) antibiotics and antimicrobials
B) broad- and narrow- spectrum antimicrobials
a. antibiotics are natural compounds produced by microbes whereas antimicrobial drugs include drugs that are partially or entirely synthesized in a lab; b. broad-spectrum antibiotics, e.g., ciprofloxacin, are effective against a wide range of gram-positive and gram-negative bacteria; whereas narrow-spectrum antibiotics may only work agains a small class of bacteria, e.g., penicillin against gram-positive bacteria
What factors influence the size of the zone of inhibition for an antimicrobial?
The size of the zone of inhibition for a particular antibiotic is influenced by diffusibility of the agent, size of the inoculum, type of medium, and resistance mechanisms of the test organism
Why are certain gram-negative bacteria more resistant than gram-positive bacteria to antimicrobials that attack cytoplasmic targets?
The outer membrane of gram-negative bacteria acts as a permeability barrier and can restrict the entry of antimicrobials into the cell
Why are gram-positive bacteria typically more resistant than gram-negative bacteria to antimicrobials that disrupt plasma membranes, such as polymyxin B?
Gram-positive bacteria have thick cell walls that maintain cell stability even when the plasma membrane is disrupted by polymyxin B
If bacterial isolate shows intermediate to moderate resistance to an antimicrobial, how might this drug still be successfully used in the treatment of this microbe?
Even when dealing with an organism with intermediate or moderate resistance to an antibiotic, increasing the dosage of the antibiotic may effectively counteract the resistance and successfully treat the infection
What specific medium must be used in testing the effectiveness of antimicrobial drugs?
Mueller-Hinton medium
If an infectious agent is sensitive to several antimicrobial drugs, what other considerations might be used to determine the best treatment option?
Therapeutic index of the agent, cost, other patient health issues, allergies, etc.
Differentiate between antiseptic and disinfectant.
Antiseptics, such as alcohol or iodine, are used on living tissues (e.g., skin). Disinfectants, such as bleach, are used on inert surfaces (e.g., tabletops). Most antiseptics (e.g., alcohol, iodine, peroxide) can be used as disinfectants, but most disinfectants are not recommended for use on skin (e.g., bleach, Lysol, ammonia)
What factors influence the size of the zone of inhibition produced by a chemical?
The size of the zone of inhibition is influenced by the molecular weight of the agent and its rate of diffusion in the agar.
How might the physical differences between gram-positive and gram-negative bacteria contribute to differences in chemical resistances?
Gram-negative bacteria have outer membranes that may limit penetration of chemicals that harm cytoplasmic targets
What is the importance of inoculating a control plate in lethal temperature experiments?
The control plate with bacteria not exposed to high temperatures is used for comparison and to ensure that the bacteria are capable of growth
To measure the culture temperature, why is the thermometer placed in a tube separate from the culture?
If the thermometer was in a culture tube it would interfere with the subsequent transfers of culture to plates across the time intervals and be a source of contamination
Bacillus megaterium has a high thermal death point and a long thermal death time, but is not classified as a thermophile. Explain.
B. megaterium is a mesophile and in its vegetative state it would be killed at high temperatures. The endospores it produces, however, are extremely heat resistant (thermoduric) and allow the bacteria to survive until plating and incubation at an optimal temperature
Give three reasons why endospores are much more resistant to heat than are vegetative cells,
(1) tough spore coat; (2) low moisture content; (3) large amount of calcium and dipicolinic acid
List four diseases caused by spore-forming bacteria
anthrax, tetanus, botulism, gas gangrene
Would heating the culture in a sealed, small-diameter tube that is totally immersed in a water bath produce more accurate results than the use of tubes that are partially submerged in a water bath?
Using totally immersed tubes is preferable because samples would be uniformly exposed to the prescribed temperature. In partially immersed tubes, a temperature gradient can occur between the portion of the tube immersed in the water bath and tube portion above the water level
Alexander Fleming
noticed that Penicillium, a fungus, inhibited growth of the bacterium, Staphylococcus aureus when both organisms were present on the same agar plate
