Antibiotic resistance disc tests

Dicker FI, Machka K, Braveny I (1981) The uses and limitations of disc diffusion in the antibiotic sensitivity testing of bacteria. Infection 9 18-24 Elo FI, Matikainen J, Peittari E (2007) Potent activity of the lichen antibiotic (+)-usnic acid against clinical isolates of vancomycin-resistant enterococci and methicillin-resistant Staphylococcus aureus. Naturwissenschaften 94 465 68... 

Shown in Table II are data from experiments where alate aphids were given a choice in a petri dish between two leaf discs of NC 2326 and two leaf discs of one of the test entries in Table I. Compared to NC 2326, TI 1112, 1-35, TI 1024, NFT, Red Russian, TI 1223, and TI 1068 had significantly fewer alates after 24 hours and fewer nymphs deposited after 72 hours. However, in nochoice feeding tests, apterous aphids on TI 1112, 1-35, and TI 1024 had similar survival and reproductive rates relative to NC 2326. Both survival and reproduction were reduced on NFT, Red Russian, TI 1223 and TI 1068 in nochoice feeding experiments. Jackson et al (75) proposed that the major mechanism of resistance of the nonsecreting entries is nonpreference by alate aphids, and that the entries with cuticular chemistries similar to TI 1223 and TI 1068 have both nonpreference and antibiotic components which contribute to their resistance. 

The ability to detect ESBLs has clinical relevance because it will have an impact on antibiotic therapy. Producers of ESBLs are suspected when susceptibility testing indicates resistance to ceftazidime (but may be paradoxically susceptible to ceftriaxone and cefotaxime by the minimal inhibitory concentration [MIC] or disc diffusion) and susceptible to the cephamycins (11). More recently, a breakpoint of 2 ig/mL to cefpodoxime has been suggested as a standard. Another method of detection is the use of the E-test strip with a gradient of ceftazidime on one side and ceftazidime in combination with clavu-lanate on the other. The ESBLs are thought to be present when enhancement of inhibition is demonstrated. Table 2 highlights some of the more common resistance mechanisms for antimicrobial agents used commonly in the treatment of nosocomial pneumonia. 

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