The Microbiology Laboratory
Keith Struthers in Clinical Microbiology, 2017
Here, cellulose discs impregnated with a standard amount of antibiotic are placed on agar plates inoculated with the organism to be tested. As the bacteria multiply during overnight incubation, the antibiotic diffuses into the agar. A zone of inhibition is produced proportional to the susceptibility of that organism to that antibiotic (Figures 5.31, 5.32). Using standardized criteria, the size of the zone of inhibition equates to the organism's resistance or sensitivity to the antibiotic. Results are usually reported as ‘resistant’ or ‘sensitive’; occasionally the term ‘intermediate’ may be used. Note that in the examples given here, isolate B is resistant to amoxycillin. This is due to production of β-lactamase such as the TEM1 and SHV1 of Escherichia coli and Klebsiella pneumoniae.
An Overview of Vetiveria zizanioides (Linn.) Nash (Poaceae)
José L. Martinez, Amner Muñoz-Acevedo, Mahendra Rai in Ethnobotany, 2019
The antibacterial activity is measured by zone of inhibition (mm). Totally four bacterial strains (two gram positive S. aureus, B. subtilis and two gram negative bacteria P. aeurogenosa, E. coli). Ethanolic extract of Vetiveria Zizanioides is known to posses flavonoids, alkaloids, terpenoids, saponins, tannins and phenols which, either individually or through combination, exert antimicrobial activity. The study showed that EEVZ inhibited gram negative bacteria than grampositive bacteria. Flavonoids are found to be effective antimicrobial substance against a wide range of microorganisms, probably due to their ability to complex with extra cellular and soluble proteins and to complex with bacterial cell wall; more lipophilic flavonoids may disrupt microbial membrane. Antibacterial activity of tannins may be related to their ability to inactivate microbial adhesion enzymes and cell envelope transport proteins; they also complex with polysaccharides. The presence of tannins present in the roots of Vetiveria zizanioides implied that tannin may be the active compound which may be responsible for in vitro antibacterial activity in this study. Tannin in the plant extract was found to possess antibacterial activity (Devi et al. 2010).
Natural Products from the Amazon Region as Potential Antimicrobials
Mahendra Rai, Chistiane M. Feitosa in Eco-Friendly Biobased Products Used in Microbial Diseases, 2022
A more common fruit in tropical regions, including Amazonia, is the cashew (Anacardium occidentale - Anacardiaceae). Several pharmacological activities have already been described, such as anti-inflammatory, antidiabetic and antimicrobial. Leaf plant extracts of the Anacardium occidentale L. species have shown to be potentially active against Escherichia coli, Staphylococcus aureus and Candida albicans strains, showing the antimicrobial potential of the cashew extract (Anand et al. 2015; Ajileye et al. 2015). The inhibition halo assay is another very useful method to analyze the potential antimicrobial activity. A paper disk containing an amount of a known antibiotic is put in a microorganism growth media, such as agar (Table 2.2). The zone of inhibition around the antibiotic-containing disk indicates the sensitivity of the bacteria to the antibiotic in the disk. With this assay, unknown substances or complex mixtures, such as extracts, can be evaluated in comparison to known antibiotics.
Quality and suitability of antimicrobial discs: theoretical and practical sources of error and variability
Published in Expert Review of Molecular Diagnostics, 2020
The use of antimicrobial discs remains one of the main methods for assessing antibiotic activity. The method, also known as the Kirby–Bauer test, relies on the diffusion of active ingredients from a paper disc into an agar plate that has been inoculated with the microbial sample. As the bacterial growth becomes visible a zone of inhibition appears where the antibiotic has been of sufficient concentration to inhibit growth. In clinical use, usually around six or more antibiotics are tested for a particular infection, and the zone measurements are used as a guide to decide which antibiotic to use. Various organizations (EUCAST in Europe and CLSI in the US) have published breakpoint tables of the zone sizes indicating whether the organism is resistant, intermediate, or susceptible to the antibiotic. Breakpoints roughly correlate with minimum inhibitory concentrations (MIC) though the graphs show a fair amount of scatter [1]. In addition, quality zone ranges have been published to enable the quality of discs to be checked.
Hepatoprotective potentials of methanolic extracts of Roselle and beetroots against carbon tetrachloride and Escherichia coli induced stress in Wistar rats
Published in Egyptian Journal of Basic and Applied Sciences, 2022
Kolawole Banwo, Samuel Oduola, Micheal Alao, Abiodun Sanni
Methanolic extracts of Roselle and beetroots were screened for antibacterial activities against E. coli using the agar well diffusion assay. Briefly, the diluted inoculum of E. coli (0.1 mL) was adjusted to 0.5 McFarland standard (approx. 1.5 × 108 CFU/mL) and seeded on Petri dishes containing Mueller Hinton agar (CM0337 – Oxoid, Basingstoke, United Kingdom). Wells of 6-mm diameter were punched onto the agar medium with a sterile cork borer under aseptic conditions, and the wells were filled with 100 µL of varying concentration of the plant extracts. Gentamicin was used as a positive control, while 10% Dimethyl sulfoxide (DMSO) was used as a negative control. The plates were kept at 4°C for 15 min for diffusion and then incubated for 24–48 h at 37°C. Antimicrobial activity was evaluated by measuring the zone of inhibition against the test microbe [17].
Exploiting antidiabetic activity of silver nanoparticles synthesized using Punica granatum leaves and anticancer potential against human liver cancer cells (HepG2)
Published in Artificial Cells, Nanomedicine, and Biotechnology, 2018
Rijuta G. Saratale, Han Seung Shin, Gopalakrishnan Kumar, Giovanni Benelli, Dong-Su Kim, Ganesh D. Saratale
Initially, the antimicrobial activity of the PGE and synthesized AgNPs were tested against Gram-positive and Gram-negative bacteria including Staphylococcus aureus and Escherichia coli by the standard agar disc diffusion method [34]. Each individual culture (100 μL) was uniformly spread onto the Luria–Bertani agar plates using glass rods. Then, 25 μL of PGE (control) and 25 μL of AgNPs were loaded on each sterile Whatman no. 1 paper disc (5 mm diameter) placed on each plate. The combined in vitro antibacterial activity of the AgNPs and antibiotics, vancomycin, streptomycin, oxytetracycline, gentamicin and amoxicillin against E. coli and S. aureus were also evaluated by further impregnating the discs with 25 μL of the synthesized AgNPs. Standard sterile antibiotic discs were used as the positive control. The plates were incubated at 37 °C for 24 h and the bactericidal activity evaluated by the size of the clear zone. The greater the zone of inhibition, the greater is the bactericidal activity. The synergistic antibacterial activity was documented by calculating the increase in fold area of the zone of inhibition using the following formula: A” and “B” refer to the zones of inhibition for antibiotic alone and AgNPs impregnated with antibiotics, respectively.
Related Knowledge Centers
- Auxin
- Bacteriostatic Agent
- Biological Target
- Bioprospecting
- Drug Discovery
- Microbiology
- Minimum Inhibitory Concentration
- Molecule
- Microbiological Culture
- Mechanism of Action