ANTIMICROBIAL SUSCEPTIBILITY TESTING (AST)

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Antimicrobial susceptibility testing (AST) is an in vitro technique or test carried out to determine the susceptibility of isolated bacterial pathogen to antimicrobial agents usually prior to the prescription of antibiotic therapy. It is used to determine which specific antibiotics a particular bacterium or fungus is sensitive to. Antimicrobial susceptibility testing is a laboratory method used to determine the susceptibility or resistance of bacteria or fungi to antibiotics. AST is carried out in vitro but it helps to predict the in vivo success or failure of antibiotic therapy. Susceptibility tests are performed in vitro, and it is used to measure the growth response of an isolated organism to a particular drug or drugs. One of the most important goals of AST is to detect the possible antibiotic resistance in common clinical bacterial or microbial pathogens and to in the process, assure susceptibility of the pathogen to some array of antimicrobial agents of choice for particular infections. AST informs the physician on the appropriate choice or selection of drugs to use for a particular infection or disease. When a pathogenic microorganism is isolated from the specimen of a patient, the clinical microbiology laboratory will often carry out in vitro antimicrobial susceptibility testing (antibiogram) on the isolated pathogen in order to determine the antimicrobial susceptibility profile of the microbe to a range of antibiotics/drugs using antibiotic susceptibility disks, so that treatment can be appropriately guided.

Drug or antimicrobial susceptibility testing (AST) helps to provide both the physician and microbiologist with reliable data that can confidently predict the in vivo effectiveness/efficacy of the drug in question. This procedure when undertaken is very essential to proper therapy and adequate management of the patient. Antimicrobial susceptibility testing can show which drugs are most effective against a given pathogen, and it also helps to give an estimate of the adequate therapeutic dose required to fight the pathogen.It guides physicians in the choice of appropriate drugs/antibiotics for therapy. Susceptibility testing in the clinical microbiology laboratory is usually undertaken based on diffusion or dilution techniques. The diffusion techniques makes use of paper disk impregnated with antimicrobial agents, while the dilution techniques usually make use of the powdered form of the drug in question that is serially diluted and tested on a given pathogen in order to determine the minimum inhibitory concentration (MIC) of the agent. The disk diffusion technique measures the zone of inhibition of microbial growth caused by the drug in millimeters (mm). Regular antimicrobial susceptibility tests in the clinical microbiology laboratory is crucial because it helps to spot and control resistant microbes, give an up-to-date information on the best drug for empirical treatment and routine antibiogram for suspect pathogens helps to prevent the emergence and spread of resistant genes in an hospital environment.

Antibiogram is the pattern of sensitivities of a given pathogenic microorganism towards an array of antibiotics (that are usually impregnated in a paper disk). The term “antibiogram” can be used synonymously with antimicrobial susceptibility testing (AST). Antibiogram helps the microbiologist or clinician to forecast the possibility of treating an infectious disease successfully using a particular drug or an array of antimicrobial agent. Values of an antibiogram helps to prevents the emergence and spread of drug resistant microorganisms within the hospital and community because the results of AST guides the physician or health personnel on the choice of antimicrobial therapy to proffer for the sick patient. Antimicrobial susceptibility testing should not be carried out on commensals, contaminants or normal flora of the host patients including organisms that has no relationship whatsoever with the disease process of the patient. This is why it is critical to do a proper identification of the isolated organism in order to ensure that they are not commensals but pathogens responsible for the disease process.  Susceptibility testing should only be evaluated on pathogens or pure cultures of microorganisms that have been considered to be causing the infectious disease, and this can only be actualized if the proper identification and characterization tests are carried out – in a bid to isolating only pure cultures from the patient’s specimen. Antibiogram will be misleading or useless to perform if adequate identification tests are not carried out prior to the susceptibility test.

NOTE: Therapeutic dose is defined as the drug level that is required for clinical treatment of a particular infection.

The goal of antimicrobial drug susceptibility testing as aforementioned is to predict the in vivo success or failure of antibiotic therapy.  Antimicrobial drug susceptibility testing which also means sensitivity tests, are usually performed in vitro, and the growth response of an isolated pathogen to a particular drug or drugs is measured. Sensitivity tests are performed under standardized conditions so that the results are reproducible. The test results when combined with clinical information and experience should be used to guide antibiotic choice, in order to select the most appropriate drug that will be most efficacious against a given pathogen. Laboratory tests for antimicrobial drug susceptibility testing are often performed in one of the following scenarios:

  • In infections where eradication of the infecting pathogen require drugs that are rapidly cidal (killing),
  • When a drug – resistant pathogen is isolated, and
  • When the infection is fatal unless treated specifically with the right type of drugs.

MAJOR REASONS FOR DOING AST

It is a fact that when a particular pathogen has been isolated and identified as the sole causative agent of a particular infection, clinicians/physicians often select drugs for treatment based on prior or current clinical experiences with certain infections and pathogens. Nevertheless, antimicrobial drug susceptibility testing should not be overlooked or underrated in a clinical setting so as not to encourage the emergence of drug – resistant strains which often emanate from the misuse of drugs. In addition to this, antimicrobial drug susceptibility testing:

  • Encourages rational selection of antimicrobial agents.
  • Determines drug of choice for a particular infection.
  • Determines drugs to be used in synergy, especially in the treatment of mixed infections.
  • Determine the susceptibility of a given pathogen to known concentrations of a drug.
  • Determine the potency of antimicrobial agents in solutions.
  • Determine the concentration of a given drug in body fluids.
  • Discourages the development of drug – resistant strains of microbes. When drugs are appropriately administered in the right concentration and dosage, and for the right type of infection at the right time; it will be very difficult for drug – resistant strains of pathogens to emerge and spread.
  • AST also helps to prevent blind treatment in the practice of clinical medicine.

It is noteworthy that antimicrobial drug susceptibility testing should never be performed on commensal organisms or contaminants because this would mislead the physician in administering antimicrobial agents/drugs to the patient. This could result to the patient receiving ineffective and unnecessary antimicrobial therapy, causing possible untoward effects in the patient, and resistance of microbes to the administered drug.

FURTHER READING

Ashutosh Kar (2008). Pharmaceutical Microbiology, 1st edition. New Age International Publishers: New Delhi, India. 

Block S.S (2001). Disinfection, sterilization and preservation. 5th edition. Lippincott Williams & Wilkins, Philadelphia and London.

Courvalin P, Leclercq R and Rice L.B (2010). Antibiogram. ESKA Publishing, ASM Press, Canada.

Denyer S.P., Hodges N.A and Gorman S.P (2004). Hugo & Russell’s Pharmaceutical Microbiology. 7th ed. Blackwell Publishing Company, USA. Pp.152-172.

Ejikeugwu Chika, Iroha Ifeanyichukwu, Adikwu Michael and Esimone Charles (2013). Susceptibility and Detection of Extended Spectrum β-Lactamase Enzymes from Otitis Media Pathogens. American Journal of Infectious Diseases. 9(1):24-29.

Finch R.G, Greenwood D, Norrby R and Whitley R (2002). Antibiotic and chemotherapy, 8th edition. Churchill Livingstone, London and Edinburg.

Russell A.D and Chopra I (1996). Understanding antibacterial action and resistance. 2nd edition. Ellis Horwood Publishers, New York, USA.

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