Question

“Sample”	“Germicide”	“Zone of Inhibition (mm)”	“Relative Effectiveness”
“Skin”	“10% Bleach”	15 mm	yes effective
	“70% Isopropyl Alcohol”	6 mm	effective but minimal
	“Hibiclens™”	13 mm	yes effective
	“Control”	0	no
“Nose”	“10% Bleach”	11 mm	yes effective
	“70% Isopropyl Alcohol”	7 mm	effective but minimal
	“Hibiclens™”	12 mm	Yes effective
	“Control”	0	no
“Throat”	“10% Bleach”	20 mm	yes effective
	“70% Isopropyl Alcohol”	6 mm	somewhat effective
	“Hibiclens™”	16 mm	yes effective
	“Control”	no	no
“Shoe”	“10% Bleach”	20 mm	Yes, effective
	“70% Isopropyl Alcohol”	6 mm	effective but minimal
	“Hibiclens™”	16 mm	yes, effective
	“Control”	no	no

“1. Which germicide was the most effective? Was the same germicide most effective for bacteria isolated from the different sources? What could explain this finding?”

“2. Which germicide was the least effective? Was the same germicide least effective for bacteria isolated from different sources? What could explain this finding?”

“3. Were any bacteria from a specific source most resistant to the different germicides? If so, which source?”

“4. Did the dry filter paper serve as a positive or negative control?”

“5. Look up the chemical formulas for bleach, isopropyl alcohol, and chlorhexidine gluconate (the active ingredient in Hibiclens). How does each chemical achieve its germicidal properties?”

Answer 1

the three chemicals used here are 10% bleach, 70% isopropyl alcohol, and Hibiclens (chlorhexidine gluconate) , soaked in filter papers and kept on the individual bacterial plates in order to see the amount of growth inhibition they induce ( by looking at the zone of inhibition) along with the dry filter paper which acts as the negative control, i.e does not lead to the formation of any zone of inhibition.

1) it can be seen that 10% bleach was the most effective with 15mm, 11mm, 20mm, and 20mm zones of inhibition respectively for samples collected from skin, nose, throat, and shoe. The diameter of the zone of inhibition found for bleach was higher than other chemicals for skin, throat, and shoe samples and slightly less but comparable to the zone of inhibition for Hibiclens  (11mm, against 12 mm for Hibiclens) in case of nose samples. This means that bleach acts in a way that can affect most of the microorganisms which are present in all the four samples and is effective in diffusing through the media outside the filter paper and inhibiting microbial growth.

2) Isopropyl alcohol was the least effective among the chemicals used with only 6mm, 7mm, 6mm, and 6mm clear zones for each of the samples. IPA was less effective than both bleach and Hibiclens in all the cases. This means that IPA has either low diffusivity or its antimicrobial activity is less potent. However, one of the most probable explanations here is the evaporation rate of alcohol from the filter paper as compared to chlorhexidine. Since alcohol evaporates faster, it has lower time and diffusion capability as compared to the other chemicals.

3) Bacteria from the nose were the most resistant to all the agents with the least diameter for clear zones ( 11mm, 7mm, and 12 mm respectively for bleach, IPA, and chlorhexidine.)

4) the dry filter paper was expected to give no clear zones, i.e. no results. This is a negative control. A positive control would have given a high positive response as compared to the test samples and is used as a check for the correctness of the experiment in cases where some of the chemicals might not give results.

5)

i) Bleach - HOCl or hypochlorite - it degrades fatty acids, forms chloramines by reacting with amino acids, and irreversibly oxidizes the -SH (sulfhydryl ) groups of bacterial enzymes. So bleach has got a very broad-spectrum antimicrobial property as a result of this.

ii) Isopropyl alcohol - C3H8O - cause membrane damage and protein denaturation. IPA is more active when used with water ( as 70% solution).

iii) Chlorhexidine gluconate - C34H54Cl2N10O14 - damage to outer cell layers, inner membranes and congealing of cytolplasm and proteins at higher concentration. This is also a very broad spectrum antibiotic