Prevalence of Phenotypic Silver Resistance in Clinical Isolates
Prevalence of Phenotypic Silver Resistance in Clinical Isolates
After approval by the Institutional Review Board, 130 different microorganism strains were randomly collected from patients admitted to a tertiary care unit at Mercy Hospital in Springfield, MO. The samples were collected in sterile collection tubes and transported to the microbiology lab. A microbial identification system (VITEK®2 Microbial Identification System, Biomerieux, Durham, NC) was used to identify species by measuring 47 biochemical substrates including carbon source utilization, enzymatic activities, and resistant patterns. The origins of the isolates include wounds, burns, sputum, and urine.
To determine if silver was microbicidal, a corrected zone of inhibition (CZOI) test was used. This test is similar to the standard zone of inhibition test, except zones are corrected to take into account the differences in shape of the hand-cut testing dressings. This test is a modification of the standard Kirby-Bauer test for antimicrobial sensitivity. Equivalent 0.1 mL of gram-positive bacteria were streaked on tryptic soy agar (Remel, Lenexa, KS) and 0.1 mL of gram negative bacteria were streaked on nutrient agar (Remel, Lenexa, KS) in 3 directions to form a confluent lawn. Silver dressing or control gauze was cut into 0.5 inch x 0.5 inch squares and placed in the center of each lawn. All plates were incubated for 24 hours at 37° C (gram positive) and 26° C (gram negative). The CZOI was then determined by measuring the zone of clearing across 1 direction and subtracting the width of the dressing. This was repeated across the other direction and values were averaged. The CZOI reflects only the width of clearing around the dressing. Corrected zone of inhibition tests were conducted in duplicate. To confirm that silver dressing killed the microorganism under the dressing and did not simply hinder bacterial growth due to pressure, a culture was taken from underneath each silver dressing, plated on appropriate agar plate, and allowed to grow overnight at 37° C.
The data were screened prior to analysis for accuracy and normality. Descriptive and other univariate statistics were investigated and reported.
Methods
Samples
After approval by the Institutional Review Board, 130 different microorganism strains were randomly collected from patients admitted to a tertiary care unit at Mercy Hospital in Springfield, MO. The samples were collected in sterile collection tubes and transported to the microbiology lab. A microbial identification system (VITEK®2 Microbial Identification System, Biomerieux, Durham, NC) was used to identify species by measuring 47 biochemical substrates including carbon source utilization, enzymatic activities, and resistant patterns. The origins of the isolates include wounds, burns, sputum, and urine.
Microbiology
To determine if silver was microbicidal, a corrected zone of inhibition (CZOI) test was used. This test is similar to the standard zone of inhibition test, except zones are corrected to take into account the differences in shape of the hand-cut testing dressings. This test is a modification of the standard Kirby-Bauer test for antimicrobial sensitivity. Equivalent 0.1 mL of gram-positive bacteria were streaked on tryptic soy agar (Remel, Lenexa, KS) and 0.1 mL of gram negative bacteria were streaked on nutrient agar (Remel, Lenexa, KS) in 3 directions to form a confluent lawn. Silver dressing or control gauze was cut into 0.5 inch x 0.5 inch squares and placed in the center of each lawn. All plates were incubated for 24 hours at 37° C (gram positive) and 26° C (gram negative). The CZOI was then determined by measuring the zone of clearing across 1 direction and subtracting the width of the dressing. This was repeated across the other direction and values were averaged. The CZOI reflects only the width of clearing around the dressing. Corrected zone of inhibition tests were conducted in duplicate. To confirm that silver dressing killed the microorganism under the dressing and did not simply hinder bacterial growth due to pressure, a culture was taken from underneath each silver dressing, plated on appropriate agar plate, and allowed to grow overnight at 37° C.
Descriptive Analysis
The data were screened prior to analysis for accuracy and normality. Descriptive and other univariate statistics were investigated and reported.
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