(3) The drug affects the bacterial cell's DNA, causing production of
abnormal protein.
b. A drug or antibiotic that inhibits the reproduction of pathogens is called
bacteriostatic. These drugs slow down the growth of an infection and give the natural
defenses of the body a better opportunity to combat the infection on their own. They
work in two ways. They may affect the function of the bacterial ribosomes, resulting in a
reversible disruption of protein synthesis or they may block steps in the bacteria's
metabolic pathways that are essential to the life of the microorganisms.
3-3.
BACTERIAL RESISTANCE TO ANTI-INFECTIVE AGENTS
Bacteria are constantly becoming resistant to anti-infective agents. This is one of
the major reasons that new antibiotics are continually appearing on the market. As
bacteria encounter a drug, they may begin forming new enzymes that destroy the drug
more rapidly, making the drug ineffective. Such an enzyme is beta lactamase that
deactivates such agents as penicillins and cephalosporins. Phosphorylating and
acetylating enzymes, which change the structure of a drug, may render gram-negative
bacteria resistant to certain antibiotics. Additionally, when an agent is used over an
extended period, changes in the bacterial cell wall may make it less permeable to the
drug. If the drug blocks one of the steps in the bacteria's metabolic pathway, the
bacteria may develop an alternate pathway which by-passes the block, just as a road
detour can bypass an area where a road is blocked. There are also numerous other
ways in which microorganisms can develop resistance to antibiotics. Over the past
several decades, early abuse of antibiotics when they were not really needed and
inappropriate therapy have contributed to the development of resistance. The prudent
use of antibiotic therapy and the appropriate selection of agent and type of therapy can
minimize the development of resistant strains of bacteria.
3-4.
ANTIBIOTIC SELECTION
When selecting an anti-infective agent, it would seem that the easiest thing to do
would be to use the antibiotic with the broadest spectrum. In fact, the best approach in
therapy is to select the agent that is the most effective against the specific bacteria
causing the infection. In order to do this microscopic examination, staining, or culturing
in the laboratory can identify the organism. When an organism is cultured, it is also
possible to determine its sensitivity to different antibiotics, as well as the antibiotic
concentration required for effectiveness, the minimum inhibitory concentration (MIC). In
this way, the most effective agent can be selected. Because a culture and sensitivity
(C&S) takes several days to perform, the physician will usually evaluate several factors,
such as patient condition, specific symptoms, and other similar cases which have been
seen in ma king an initial antibiotic selection. When the C&S is available, the initial anti-
biotic may be changed to one that is more effective, or if the patient's response has
been satisfactory, the original selection may be continued.
MD0808
3-3