Susceptibility Testing Flashcards
List the B-lactam antibiotics:
Penicillins
Cephalosporins
Carbapenems
Monobactams
What is the mechanism for microbial attack by the B-lactam antibiotics and the bacterial strategies for resistance, including PBPs?
During cell wall synthesis the ptdg. layer is produced by cross linking peptide bonds this is all done by the enzyme transpeptidase or by PBPs. The beta-lactam is a 4 membered ring that takes the place of the normal structure in transpeptase reaction disrupting the formation of the cell wall.
List the aminoglycoside antibiotics:
Streptomycin Gentamicin Neomycin Kanamycin Amikacin Tobramycin
Discuss the mechanism for microbial attack by the aminoglycoside antibiotics:
During the replication process mRNA is translated between the 30s and 50s ribosomal subunits. 30s is to scan the mRNA for the start codon, 50s is for the pre initiating compound.
Amino glycosides binds to the 16s ribosomal until at the A binding site in the 30s unit which prevents the docking of aminoacyl to transfer RNA, causing a misread and disruption of protein synthesis.
Discuss the agents that interfere with DNA synthesis, RNA synthesis and folic acid metabolism:
Dihydropteroate syntheseas
Dihydropteroate reductase
Interference:
- Sulfamethoxazole
- UTIs inhibit formation of synthase
- Trimethoprim-sulfamethoxazole
- Chronic UTIs prevents reductase from recycling enzymes
Define 1st, 2nd and 3rd generation cephalosporins:
The b-lactam ring is really a 6 membered dihydrothiazine ring, which are more resistant that 5-member rings. It has modifiable sites to increase action on bacteria. Alterations of positions R₁ and R₂ have made the differences in 1-2 and 3rd generations.
What effect does a bacteriostatic antibiotic have on the microbe?
Inhibits microbial growth, does not kill it
What effect does a bactericidal antibiotic have on the microbe?
Antimicrobial agent kills the microbe
Examine what is meant by CLSI recommendations for selective reporting for Group A and the antibiotics included in the group:
Those animicrobial agents that are used and reported routinely.
Ampicillin, cefazolin, cephalothin, gentamicin
Examine what is meant by CLSI recommendations for selective reporting for Group B and the antibiotics included in the group:
Group B reported only if an organism shows resistance to A agents, patients cannot tolerate A agents, the infection not responding or it is a multi organism infection
Mezlocillin, ticarcillin, amoxicillin group, cephalosporins other than ceftazidime, amikacin, T/S.
Examine what is meant by CLSI recommendations for selective reporting for Group C and the antibiotics included in the group:
Broad-spectrum activity, orgs. that are resistant to A and B agents. Ceftazidime, kanamycin, netilmicin, tobramycin, tetracycline, chloramphenicol
Examine what is meant by CLSI recommendations for selective reporting for Group D and the antibiotics included in the group:
Urine antimicrobials
Cinoxacin, loracarbef, nitrofurantoin, sulfisoxazole, trimethoprim
How do antibiotics of the quinoline group affect the microbes? List the members of this group:
Inhibition of DNA gyrate by binding to the A subunit of DNA gyrate, the enzymes needed for DNA replication are Topoisomerases I, II, III, IV
Quinolone specifically targets the topoisomerase I and IV and prevents supercoiling by inhibiting DNA synthesis.
Nalidixic acid, cinozacin, farehoxacin, Ciprofloxacin
What is the purpose of an MBC?
For immunosuppressed pts. or very serious infections, the synergistic combination of the antimicrobial and the patients own immune system is to weak and insufficient to inhibit the microbe so bactericidal levels are needed.
Discuss how to calculate an MBC:
If the number of colonies from the subculture plate total less than 0.1% of the initial inoculum.
- from growth well dilute org. to 1:1000 (0.01 ml in 10 ml saline)
- 0.01 ml aliquot is plated from a clear well
- 18 hrs - growth plate = # of colonies x 10⁴
- test dilutions = # of colonies x 10⁴
The MBC endpoint is calculated if 99.9% killing (0.1% survival) has occurred
List the important criteria for Kirby-Bauer disk diffusion susceptibility testing:
Lawn correct conc. of microbe on media
select appropriate disks
Press lightly on surface within 15 min. of inoc.
incubate for 16 to 18 hrs
No CO₂ unless orgs has to have it
Standardized depth of media
Define susceptible, intermediate and resistant:
A regression analysis plot is used by CLSI and US food and drug admin. to develop zone interpret. criteria.
Susceptible-antimicrobial will most likely be effective at standard dose
Intermediate-antimicrobial is not likely to be effective at standard dose
Resistant-antimicrobial will most likely be ineffective
What is an E-test?
Standardized conc. of bacteria is lawned on media. A plastic test strip with antimicrobial on back is placed on inoculated media. an elliptical inhibitory area is formed around each strip. the MIC is determined where the growth ellipse intersects the E test strip
List the times that an E-test is frequently used:
Anaerobes, fastidious orgs.
Define MRSA, how it is tested, and its implications for the patients:
MRSA, some isolates are resistant to multiples agents (aminoglycosides), vancomycin is then the drug of choice for MRSA with life threatening infections. The pt. is isolated and control of the microbe is critical.
MRSA penicillinase resistant penicillins is due to the PBP MeC 2a or Mec 2’ on the cells.
Oxacillian resistant
Discuss VRE, how it is tested, and its implications for the patients:
Automated systems are not acceptable for vanc resistance check. The drug is incorporated into agar and growth vs. no growth is observed.
Why are disk diffusion plates not incubated under increased carbon dioxide atmosphere?
Unless specifically recommended for a particular fastidious microbe, ambient atmosphere should be used because CO₂ lowers the pH, which affects the activity of some antimicrobial agents.
A zone of 80% or more inhibition is considered susceptible with which anti-microbial agent:
Sulfonamides, trimethoprim, and trimethoprim/sulfamethoxazole
What is the expected susceptibility pattern for the following organisms?
a. Klebsiella = amp / ceph
Amp (res)
Ceph (sens)
