Antibiotics and Antibiogram Antibiotics What are antibiotics natural
Antibiotics and Antibiogram
Antibiotics
What are antibiotics? Ø natural or synthetic substance Ø therapeutic activity at very low specific dose against bacteria Ø with the properties: ü to kill bacteria: bactericidal effect ü to limit bacterial growth: bacteriostatic effect Notes ü Antiseptics are not antibiotics ü Their function is to kill plenty of germs (bacteria, fungi, viruses). ü Their mode of action is not specific ü They are used locally for external application
Some history 1877: Pasteur and Joubert observed low bacterial culture in the presence of mold 1897: Duchesne described the inhibition of bacterial culture by a fungus Penicillium 1929: Sir Alexander Fleming discovered penicillin watching its bacterial cultures inhibited by Penicillium notatum 1939: Chain got a form stable of penicillin 1942: Industrial Production of penicillin
Characteristics of antibiotics Selective toxicity: kill or inhibit the pathogenic germs causing as little damage as possible to the patients Systemic (mostly) Act in the body after passing through the bloodstream Effectiveness: Ø Narrow spectrum limited to a variety of microorganisms Ø Wide spectrum, acting on many different types of pathogenic agents Bactericidal effect (death of the target organism) or bacteriostatic effect (reversible inhibition of growth)
Modes of antibiotics action inhibition of the synthesis of the bacterial cell wall Bacitracin Fosfomycin b-lactams (Penicillins, Cephalosporins. . . ) Glycopeptides inhibition of the synthesis of the cytoplasmic membrane inhibition of protein synthesis Aminoglycosides Lincosamide Macrolides Tetracyclines Colistin Polymixin B inhibition of DNA synthesis Other mechanisms For Folic acid synthesis: • Trimethoprim • Sulfonamides Quinolones Nitrofurantoin Nitrimidazole
Minimum inhibitory concentration: MIC Lowest antibiotic concentration needed to inhibit the growth of bacteria MIC = measuring a concentration Ø To determine the MIC of an antibiotic we have to test the effect of increasing concentrations of antibiotic on the growth of a germ Ø The conventional method is by dilution in liquid medium MIC
Antibiogram
Antibiogram - Définition Ø It is the measurement of MIC of several antibiotics against an isolated germ. Ø It is the sensitivity profile of a bacterium. For each antibiotic, one MIC for a specific germ For each germ, one antibiogram for a group of antibiotics Ø The realization of antibiogram is essential to determine the most suitable antibiotics to fight against bacterial infection
Antibiogram - Solid media realization Ø Agar medium previously inoculated with a pure strain in standard conditions: ü Muller Hinton ü Thickness 4 mm ü Standardized inoculum ü Incubation: 18 to 24 hour at 37°C Ø Antibiotic impregnated papers deposited on the agar Ø Antibiotics diffuse into the agar: concentrations are inversely proportional to the distance of the disc
Antibiogram – Solid media results Ø After incubation, the disks are surrounded by circular inhibition zones corresponding to an absence of culture. Ø The diameter of the zone is measured. Ø The MIC is determined
Antibiogram - Interpretation The lower critical concentration (c) represents the minimum blood concentration obtained in a patient following administration of a recommended dose of antibiotic. The upper critical concentration (C) represents the maximum permissible blood concentration in a patient following administration of a maximal dose of antibiotic. This is also the antibiotic toxicity threshold. Each of these concentrations corresponds to a diameter around the disk for antibiotic susceptibility testing in agar (d and D). Intermediate Sensitive MIC Diameter around the ATB disc Resistant MIC c C D d ATB Concentration
Antibiogram - Interpretation Using Abacus Ø Standardized method Ø So possibility of using reading abacuses Ø Direct interpretation of the result
Antibiogram – Other methods Etest® Ø Etest® consists of a predefined gradient of antibiotic concentrations on a plastic strip and is used to determine the Minimum Inhibitory Concentration (MIC) of antibiotics. Ø Direct reading of the MIC value on the strips
Antibiogram – Other methods Ø Method with two critical concentrations Culture Resistant Culture with c but no culture with C Intermediate No culture sensitive C c Ø Automated methods Vitek 2 of Biomérieux, Phoenix of Becton Dickinson, Microscan Walkaway of Siemens The VITEK 2 system quickly determines (~ 4 hours) minimum inhibitory concentrations (MIC) by analyzing the bacteria growth kinetics with the presence of different concentrations of antibiotics.
Antibiotic resistance
Antibiotic resistance - Introduction Ø A very important therapeutic progress in the twentieth century Ø Increased life expectancy of more than ten years But Ø the widespread use of many antibiotics, including preventive treatment in veterinary and human medicine, Ø use as a food supplement in animal feed, Ø use for the treatment of plants, led to the development of antibiotic-resistant microorganisms populations and a general decline in therapeutic efficacy.
Antibiotic resistance - Some numbers Antibiotic Year of introduction First appearance of resistance Sulfamides 1936 1940 Pénicilline G 1943 1946 Streptomycine 1943 1959 Chloramphénicol 1947 1959 Tétracycline 1948 1953 Erythromycine 1952 1988 Ampicilline 1961 1973 Ciprofloxacine 1987 2006
Antibiotic resistance – Natural or Acquired Bacterial resistance Acquired resistance Some strains of the species Tends to spread Natural resistance ü Fixed ü All the strains of a species ü Specific for an ATB ü chromosomal ü Transmissible vertically Mutation 20% ü Spontaneous ü Stable ü Specific for an ATB ü Chromosomal ü Unusual ü Transmissible vertically Extra. Chromosomal 80% ü Contagious ü Nonspecific: multidrugresistance ü External genetic material ü Frequent ü Transmissible horizontally
Antibiotic resistance - Mechanisms Ø Modification of the target of the antibiotic via mutation or by an enzyme Ø Production of an enzyme that Ø The waterproofing of membrane of the bacteria or the efflux of antibiotics modifies or destroys the antibiotic
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