MICB 211 Chapter 13 Notes

Therapy and Prevention of Bacterial Diseases

Chemotherapeutic Agents

-chemical compounds used to combat infectious diseases

-used internally

-selective toxicity; compounds inhibit or kill bacteria without causing harm to the host

-sensitivity of microorganisms to antibiotics and other chemotherapeutic agents varies

Optimal Attributes

-solubility in body fluids with good penetration to infection site

-effectiveness against bacteria at a low concentration

-a broad spectrum of activity

• ex. activity against a broad range of bacteria

-low frequency of resistant development

-low rate of breakdown of resistance development

-low rate of breakdown or excretion in the body

-low toxicity

-well tolerated in human body

-non allergenic

Antiseptic

• chemical agents that are applied to living tissues to kill or inhibit the growth of microorganisms

• compounds used for washing hands or treating surface wounds

Disinfectants

• chemical agents used on inanimate objects to kill microorganisms

• agents that are too toxic to be used inside the body

Antibiotics

-chemotherapeutic agents that are able to kill or inhibit growth of bacteria

-initially differentiated from synthetic compounds

• because they were natural products derived from microbial activity

-modern day antibiotics have been modified by chemical changes carried out in the laboratory

• semi-synthetic antibiotics

-in 20th century, scientists have discovered ways to make or modify antibiotics

• natural, semi-synthetic, synthetic antibiotics

-Gram(+) bacteria more sensitive to antibiotics than Gram(-)

-antibiotics don’t always possess optimal attributes of chemotherapeutic agents but they are safe and effective drugs

-must be able to inhibit bacterial growth or kill bacterial cells while not affecting the host

-inhibits or kills the growth of bacteria by different mechanisms

• primary way → targeting or inhibiting a key process in bacterial growth and/or survival

Broad-Spectrum Antibiotics

-antibiotic that acts on both Gram(+) and Gram(-) bacteria

-most frequently used in medicine

Narrow-Spectrum Antibiotics

-antibiotic that acts only on a single group of organisms

-good for m.tuberculosis

Beta-Lactam Antibiotics

-most commonly used in hospitals

-naturally derived penicillin-like cephalosporin antibiotics and their many semi-synthetic derivatives

-contains a 4-membered ring

Common Antibiotic Classes

Testing for Antibiotic Sensitivity

Disk Diffusion Assay

-most times antibiotic therapy is used without actual knowledge of the identity of the pathogen or the pathogen’s sensitivity to antibiotics

-a simple method to determine antibiotic sensitivity of a clinal isolate is through disk diffusion antibiotic susceptibility

-bacterial isolate is spread evenly onto surface of an agar plate so that when bacteria grow it will form a confluent lawn

-paper discs impregnated with various antibiotics are placed on the surface of the plate and antibiotic is allowed to diffuse into culture medium

-as bacteria on lawn grow, they are inhibited to varying degrees by antibiotic diffusing from disk

-inhibition of bacterial growth around disk following incubation indicates sensitivity to particular antibiotic contained in the disk

-size of zone of inhibition is dependent on

• diffusion rate of antibiotic

• the degree of sensitivity of microorganism

• growth rate of bacterium

-test is performed under standardized condition

Zone of Inhibition

• radius of bacteria free circular zones

• under standard conditions established for each antibiotic

• if zone of inhibition is equal to or greater than standard → organism is susceptible to antibiotic

• if zone of inhibition is less than standard → organism is resistant

Minimal Inhibitory Concentration (MIC)

-antibiotics can be classified as bactericidal (kill bacteria) or bacteriostatic (inhibit bacterial growth without directly killing)

• when concentration of bacteriostatic antibiotics fall below certain concentration (MIC) → bacteria starts to grow again

• when concentration of bactericidal antibiotics fall below MIC → bacteria can’t grow

-MIC of an antibiotic against a specific bacteria can be determined by finding the lowest concentration of antibiotic that prevents visible bacterial growth

• gold standard of determining bacterial susceptibility to an antibiotic

• used when disk diffusion has indefinite results

-to perform MIC measurement, bacteria are exposed to a range of antibiotic concentrations on either liquid or solid growth media

• antibiotic and growth conditions are case dependent

Antibiotic Resistance

-exposure to antibiotics doesn’t cause bacteria to become drug resistant

-changes in bacterium that enable it to resist the antibiotic occur naturally as a result of mutation or genetic exchange and recombination

-exposure to antibiotic selects for outgrowth of the bacteria that have become resistant through natural selection

Acquiring Resistance — Bacteria

-acquired ability of microorganism to resist antibiotic effects where it normally is sensitive

-first observed shortly after first antibiotic was introduced into clinical use in 1940s

-not all antibiotics act against all microorganisms

-some bacteria carry genetic information information (resistance genes) that renders them resistant to antibiotic they produce

• ex: may have gene that encodes an enzyme that can modify the antibiotic by breaking a bond or by adding a phosphate group

• can acquire resistance genes via horizontal gene transfer

Intrinsic Resistance

• bacteria may be naturally resistant to antibiotic because

  • they lack structures that antibiotics inhibit

    • ex: mycoplasmas resistant to penicillins because they don’t have peptidoglycan

  • organisms naturally possess non-specific efflux pumps that transport antibiotics out of the organism before it becomes effective

  • some Gram (-) bacteria are naturally resistant due to poor permeability across outer membrane and efflux & antibiotic inactivating enzymes that take advantage of the slow uptake of antibiotics into the bacterial cell

• regular or natural feature of bacteria that allows it to resist the antibiotic

Acquired Resistance

• bacteria become resistant to antibiotics by modifying the target of antibiotic

• mutations are common in bacteria that are producing the antibiotic

• mutations within bacteria that

  • change the structure of target site

  • alter a metabolic pathway

  • change permeability of outer membrane

• mutations occur naturally during DNA replication when mistakes are made that benefit bacteria in the presence of an antibiotic

• mutations are passed to progeny of cell (vertical gene transfer) during cell division to create larger pool of antibiotic resistant bacteria

Acquiring Resistance — Human Contributions

-antibiotic abuse encourages the emergence of antibiotic resistant forms

-problems with using antibiotics

• antibiotic usage in clinical practice is more often than is necessary

• incorrect doses or duration of treatment

• people don’t complete their treatment duration

  • stop taking antibiotics after they’re feeling better → bacterial are subjected to lessened doses of antibiotics and remain alive to evolve into resistant organisms

  • when using the same antibiotic in the future → kills sensitive bacteria (even useful bacteria) → resistant bacteria thrives because they have reduced competition

• hospitals are an environment that drives the antibiotic resistance

  • large doses used to prevent infection during and post operation

• antibiotics available without prescription in developing countries

• 80% of antibiotics produced used in agriculture

  • used as

    • growth promoting agents in livestock

    • prophylactic treatment of livestock, crops, farmed fish

A Global Crisis

-emergence and spread of multi-drug-resistant pathogens has increased substantially in the last 20 years

-in some cases, the only drugs available to treat a particular infection have been rendered ineffective because of antibiotic resistance

-superbugs (bacteria) are resistant to most or all types of antibiotic treatment

• MRSA - methicillin-resistant Staphylococcus aureus

• VRE - vancomycin-resistant Enterococcus

• imipenem resistant Pseudomonas aeruginosa

• penicillin resistant Streptococcus pneumoniae