1. Discussion
Disinfectants are chemical agents used on inanimate objects to lower the level
of microbes present on the object.
Antiseptics are chemicals used on living tissue to decrease the number of
microbes present in that tissue.
Disinfectants and antiseptics affect bacteria in many ways. Those that result in
bacterial death are called bactericidal agents. Those causing temporary inhibition of
growth are bacteriostatic agents.
No single antimicrobial agent is most effective for use in all situations - different
situations may call for different agents. A number of factors affect selection of the
best agent for any given situation - Antimicrobial agents must be selected with
specific organisms and environmental conditions in mind. Additional variables to
consider in the selection of an antimicrobial agent include pH, solubility, toxicity,
organic material present, and cost.
Once an agent has been selected, it is important to evaluate it's effectiveness. In
evaluating the effectiveness of antimicrobial agents, the concentration, length of
contact, and whether it is lethal (-cidal) or inhibiting (-static) at that concentration and
exposure are the important criteria.
One method of measuring the effectiveness of a chemical agent is to determine its
zone of inhibition. In the agar diffusion method, one species of bacteria is uniformly
swabbed onto a nutrient agar plate. Chemicals are placed on paper disks. These discs
are added to the surface of the agar. During incubation, the chemical diffuses from the
disk containing the agent into the surrounding agar. An effective agent will inhibit
bacterial growth, and measurements can be made to quantify the size of the zones of
inhibition around the disks. The relative effectiveness of a compound is determined by
comparing the diameter of the zone of inhibition with values in a standard table.
The agar diffusion test is not used to determined whether a chemical is bactericidal
(kills bacteria) or bacteriostatic (inhibits bacteria) - instead this characteristic is
determined by the dilution method. In this method the bacterium of interest is placed
in a tube containing the chemical which is being tested. The bacterium is then added
(subcultured) onto a nutrient agar plate. If the bacterium grows on the nutrient agar
the chemical is bacteriostatic; if not, it was killed by the chemical which is then
termed "bactericidal."

2. , the effectiveness of a chemical in sensitivity testing is based on the size of the
zone of inhibition. But the zone of inhibition varies with the relative rate of
diffusion of the agent, the size of the inoculum, the type of medium, and many
other factors. Therefore, zone size alone cannot be used to determine the absolute
effectiveness of an antibiotic, antiseptic, or disinfectant, but can certainly be used
to test relative effectiveness.
Two important rules to remember concerning the use of antiseptics and
disinfectants are (1) always use the most concentrated form that will cause the least
damage to the tissue or inanimate surface, and (2) if long-term use is necessary,
apply either a combination of agents or frequently change to another effective
agent. The continued application of a single agent (especially at low
concentrations) will select for microbial mutants that are resistant to the agent.
The risk is minimized when you use higher concentrations and is almost eliminated
when you use a combination of effective agents. These same rules apply to
antibiotics. In this experiment you will assess the relative effectiveness of certain
antibiotics, antiseptics, and disinfectants in killing bacteria. The bacteria tested
will be bacteria common in various parts of your body. One simple way to evaluate
the relative effectiveness of antimicrobial agents is to use the zone-of-inhibition
method. With this method, you apply the chemical to a freshly inoculated plate,
incubate the culture, and then look for a zone of inhibition. The presence of such a
clear zone (lack of growth) surrounding the chemical shows either the cells have
been killed or that their growth has been inhibited (but you cannot tell which). In
other words, a zone of inhibition does not discriminate between bacteriostatic and
bactericidal chemicals.

3. he bacteria of interest is swabbed uniformly across a culture plate. Then a filter-paper disk,
impregnated with the compound to be tested, is placed on the surface of the agar. The
compound diffuses out from the filter paper into the agar. The concentration of the
compound will be higher next to the disk, and will decrease gradually as distance from the
disk increases. If the compound is effective against bacteria at a certain concentration, no
colonies will grow wherever the concentration in the agar is greater than or equal to that
effective concentration. This region is called the "zone of inhibition." Thus, the size of the
zone of inhibition is a measure of the compound's effectiveness: the larger the clear area
around the filter disk, the more effective the compound. If bacteria growth was not inhibited
around this piece of filter paper, the zone of inhibition is the diameter of the filter paper. Label
the filter papers and their respective antibiotics with the smallest zones of inhibition as
"Resistant," the ones with the largest zones as "Sensitive" and those in between "Intermediate."
Read more: How to Calculate the Zone of Inhibition |
eHow.com http://www.ehow.com/how_5845724_calculate-zone-inhibition.html#ixzz1vZsAQVpQ
Read more: How to Calculate the Zone of Inhibition |
eHow.com http://www.ehow.com/how_5845724_calculate-zone-inhibition.html#ixzz1vZs2MmNb
After overnight incubation, examine your plates (keep them covered at all times) to
measure the zone of inhibition.
a. The control plates should show uniform colonies over the entire surface of the
plate. If the distribution is highly uneven, you will need to improve your
innoculation technique and repeat the experiment. The filter disks should not
impede bacterial growth, since they contained only water.
b. If your disinfectants are effective at the concentrations you tested, you should
see zones of inhibition around the disinfectant disks. The clear zones around
each disk should have a uniform diameter, since diffusion of the compounds
through the agar should be uniform in every direction. If this is not the case,
suspect either your impregnation technique, or poor contact of the filter paper
with the agar.
2. Measure the diameter of the zone of inhibition around each disk. Keeping the lid of
the plate in place, use a ruler to measure the diameter of the clear area in
millimeters. You will get four separate measurements for each dilution of each
disinfectant—one from each quarter section of the test plate. The length of this
clear zone is the zone of inhibition.
all plates should be disinfected for safe disposal.
1. The best way to dispose of bacterial cultures is to pressure-sterilize (autoclave) them
in a heat-stable biohazard bag.

4. 2. If autoclaves or pressure cookers are not available, an alternative is to bleach the
plates.
References
1. http://www.sciencebuddies.org/science-fair-projects/project_ideas/MicroBio_p013.shtml 22nd
May 2012
2. http://biology.bard.edu/ferguson/course/bio112/Lab/Lab_04_Antibiotics_Antiseptics_Disinfect
ants.pdf
New antibiotics are constantly being discovered to replace older types that no longer fight
bacteria. Unfortunately, the longer an antibiotic is used, the greater chance the harmful bacteria
will become immune to the antibiotic.
met largely by semisynthetic tailoring of natural product scaffolds discovered in
th
the middle of the 20 century. More recently, however, advances in technology
have sparked a resurgence in the discovery of natural product antibiotics from
bacterial sources. In particular, efforts have refocused on finding new antibiotics
from old sources (for example, streptomycetes) and new sources (for example,
other actinomycetes, cyanobacteria and uncultured bacteria). This has resulted in
several newly discovered antibiotics with unique scaffolds and/or novel
mechanisms of action, with the potential to form a basis for new antibiotic classes
addressing bacterial targets that are currently underexploited
The clinical need for new classes of antibiotic continues to grow, as drug resistance erodes the
efficacy of current therapies. Historically, most antibiotics were discovered by random screening
campaigns, but over the past 20 years, this strategy has largely failed to deliver a sufficient
range of chemical diversity to keep pace with changing clinical profiles. A more rational
approach to drug hunting has been greatly potentiated by the availability of bacterial genomic
information. The rapid progress in sequencing and analysis of these small, prokaryotic genomes
has enabled the concomitant development of powerful new technologies that are already
enhancing the potential utility of genomic information. The future promises versatile and precise
tools to understand what makes a successful antibiotic and moreover the means to identify and
evaluate novel classes of drug.