The rickettsias (also: rickettsiae) are parasites of
mammals (including people) and are often spread by
arthropod vectors. In the United States, the most
common rickettsial disease is Rocky Mountain spotted
fever, which is spread by ticks. In other parts of the
world, epidemic typhus, which is spread by body lice,
is still an important disease. From a historical per-
spective, until World War I, more people died of epi-
demic typhus during times of war than were killed by
weapons.
The chlamydias (also: chlamydiae) cause several
human diseases, including ornithosis (parrot fever)
and trachoma, which is the leading cause of blindness
throughout the world. In the United States, chlamy-
dial infection of the genitourinary tract has become
one of the most prevalent sexually transmitted dis-
eases, with estimates of 4 million new cases each year.
Both rickettsial and chlamydial infections can be
treated with antibiotics.
Antibiotics
Antibiotics are chemicals that are used to treat bacter-
ial infections. A broad-spectrumantibiotic is one that
affects many different kinds of bacteria; a narrow-
spectrumantibiotic affects just a few kinds of bacteria.
The use of antibiotics is based on a very simple
principle: Certain chemicals can disrupt or inhibit the
chemical reactions that bacteria must carry out to sur-
vive. An antibiotic such as penicillin blocks the forma-
tion of bacterial cell walls; without their cell walls,
bacteria will die. Other antibiotics inhibit DNA syn-
thesis or protein synthesis. These are vital activities
for the bacteria, and without them bacteria cannot
reproduce and will die.
It is very important to remember that our own cells
carry out chemical reactions that are very similar to
some of those found in bacteria. For this reason, our
own cells may be damaged by antibiotics. This is why
some antibiotics have harmful side effects. The most
serious side effects are liver and kidney damage or
depression of the red bone marrow. The liver is
responsible for detoxifying the medication, which may
accumulate and damage liver cells. Similar damage
may occur in the kidneys, which are responsible for
excreting the medication. The red bone marrow is a
very active tissue, with constant mitosis and protein
synthesis to produce RBCs, WBCs, and platelets. Any
antibiotic that interferes with these processes may
decrease production of all of these blood cells.
Patients who are receiving any of the potentially toxic
antibiotics should be monitored with periodic tests of
liver and kidney function or with blood counts to
assess the state of the red bone marrow.
Another problem with the use of antibiotics is that
bacteria may become resistant to them, and so be
unaffected. Bacterial resistancemeans that the bacte-
ria are able to produce an enzyme that destroys the
antibiotic, rendering it useless. This is a genetic capa-
bility on the part of bacteria, and it is, therefore,
passed to new generations of bacterial cells.
Let us use an example with small numbers and say
that, of 1000 bacterial cells, 998 are killed by a certain
antibiotic. The remaining two bacteria have the gene
for an enzyme that breaks the antibiotic molecule, so
they survive. What will these two bacteria do? Each
will divide by binary fission, becoming two cells, four
cells, eight cells, and so on. At the end of 20 divisions
(which under optimum conditions may take less than
24 hours) each bacterium has become more than a
million bacteria. And what is true of all of these bacte-
ria? They all have the gene for resistance to that par-
ticular antibiotic.
This is exactly what has happened with Staphylo-
coccus aureus, most strains of which are resistant to
penicillin, and some strains of which are resistant
to many other antibiotics as well. Most of the gram-
negative intestinal bacilli are resistant to a great vari-
ety of antibiotics. This is why culture and sensitivity
testingis so important before an antibiotic is chosen
to treat these infections.
Education is also important, and healthcare person-
nel must help educate the general public not to
demand antibiotics for colds or the flu. These are viral
diseases for which antibiotics are useless. What hap-
pens is that the antibiotic kills all susceptible bacteria
(even beneficial ones) and leaves resistant bacteria to
become the most numerous, perhaps to cause serious
disease in the future.
To counteract bacterial resistance, new antibiotics
are produced that are not inactivated by the destruc-
tive bacterial enzymes. Within a few years, however,
the usefulness of these new antibiotics will probably
diminish as bacteria mutate and develop resistance.
This is not a battle that we can ever truly win, because
bacteria are living organisms that evolve as their envi-
ronment changes. We must never underestimate the
ability of bacteria to evolve ways to neutralize the
antibiotics we use to kill them. The development of
new antibiotics is crucial if we are to stay a step ahead
of these remarkably adaptable microorganisms.
Antibiotics have changed our lives, although510 An Introduction to Microbiology and Human Disease