Side effects of antibiotics and preventive measures.

Microbiological basis of chemotherapy: the concept of chemotherapy,  the mechanism of action of sulfonamides. Antibiotics, methods of preparation. Complications of antibiotic therapy, their prevention. Drug resistance. Mechanisms for the development of resistance of microbes to antibiotics. Determination of the activity of antibiotics and the sensitivity of bacterial cultures to antibiotics.

The modern era of antimicrobial chemotherapy began in 1929 with
Fleming’s discovery of the powerful bactericidal substance penicillin with broad
antimicrobial activity. In the early 1940”s, spurred partially by the need for
antibacterial agents in World War II, penicillin was isolated, purified and injected
into experimental animals, where it was found to not only cure infections but
also to possess incredibly low toxicity for the animals. This fact ushered into
being the age of antibiotic chemotherapy and an intense search for similar
antimicrobial agents of low toxicity to animals that might prove useful in the
treatment of infectious disease. The rapid isolation of streptomycin,
chloramphenicol and tetracycline soon are followed, and by the 1950’s, these and
several other antibiotics were in clinical usage.

The most important property of a clinically-useful antimicrobial agent,
especially from the patient’s point of view, is its selective toxicity, i.e., that the
agent acts in some way that inhibits or kills bacterial pathogens but has little or
no toxic effect on the animal taking the drug This implies that the biochemical
processes in the bacteria are in some way different from those in the animal
cells, and that the advantage of this difference can be taken in chemotherapy.
Antibiotics may have a cidal (killing) effect or a static(growth inhibitory) effect on a
range of microbes. The range of bacteria or other microorganisms that are
affected by a certain antibiotic are is expressed as its spectrum of activity.
Antibiotics effective against procaryotes which kill or inhibit a wide range of
Gram-positive and Gram-negative bacteria are said to be broad spectrum. If
effective mainly against Gram-positive or Gram-negative bacteria, they are
narrow spectrum. If effective against a single organism or disease, they are
referred to as limited spectrum.

Antibiotics are low molecular-weight (non-protein) molecules produced
as secondary metabolites, mainly by microorganisms that live in the soil. Most
of these microorganisms form some type of a spore or other dormant cell, and
there is thought to be some relationship (besides temporal) between antibiotic
production and the processes of sporulation. Among the molds, the notable
antibiotic producers are Penicillium and Cephalosporium, which are the main
source of the beta-lactam antibiotics (penicillin and its relatives). In the bacteria,
the Actinomycetes, notably Streptomyces species, produce a variety of types of
antibiotics including the aminoglycosides (e.g. streptomycin), macrolides (e.g.
erythromycin), and the tetracyclines. Endospores-forming Bacillus spp. produce
polypeptide antibiotics such as polymyxin and bacitracin. The Table 12 is a
summary of the classes of antibiotics and their properties including their
biological sources.

Kinds of antimicrobial agents and their primary modes of action

1.            Cell wall synthesis inhibitors Cell wall synthesis inhibitors generally
inhibit some step in the synthesis of bacterial peptidoglycan. Generally
they exert their selective toxicity against eubacteria because human cells
lack cell walls.

Beta-lactam antibiotics Chemically, these antibiotics contain a
4-membered beta lactam ring. They are the products of two groups of
fungi, Penicillium and Cephalosporium molds, and are correspondingly
represented by the penicillins and cephalosporins. The beta-lactam
antibiotics inhibit the last step in peptidoglycan synthesis, the final
cross-linking between peptide chains. Beta-lactam antibiotics
are normally bactericidal and require that cells be actively growing in
order to exert their toxicity.

Natural penicillins, such as Penicillin G or Penicillin V, are produced by
fermentation of Penicillium chrysogenum. They are effective against
streptococcus, gonococcus and staphylococcus, except where resistance
has developed. They are considered narrow spectrum since they are
not effective against Gram-negative rods.

Semisynthetic penicillins first appeared in 1959. A mold produces the main
part of the molecule (6-aminopenicillahic acid) which can be modified
chemically by the addition of side chains. Many of these compounds have
been developed to have distinct benefits or advantages over penicillin G,
such as increased spectrum of activity (effectiveness against Gram-negative
rods), resistance to penicillinase, effectiveness when administered orally,
etc. Amoxicillin and have broadened spectra against Gram-
negatives and are effective orally; is penicillinase-resistant.
Clavulanic acid is a chemical sometimes added to a semisynthetic
penicillin preparation to inhibit beta-lactamase enzymes and has given
extended life to penicillinase-sensitive beta-lactams.

Although nontoxic, penicillins occasionally cause death when
administered to persons who are allergic to them. In the U.S. there are
300-500 deaths annually due to penicillin allergy. In allergic individuals
the beta lactam molecule attaches to a serum protein which initiates
an IgE-mediated inflammatory response.

Cephalolsporins are beta-lactam antibiotics with a similar mode of action
to penicillins that are produced by species of Cephalosporium. They have
a low toxicity and a somewhat broader spectrum than natural penicillins.
They are often used as penicillin substitutes, against Gram-negative
bacteria, and in surgical prophylaxis. They are subject to degradation
by some bacterial beta-lactamases, but they tend to be resistant to beta-
lactamases from S. aureus.

Bacitracin is a polypeptide antibiotic produced by Bacillus species. It
prevents cell wall growth by inhibiting the release of the muropeptide
subunits of peptidoglycan from the lipid carrier molecule that carries
the subunit to the outside of the membrane Teichoic acid synthesis,
which requires the same carrier, is also inhibited. Bacitracin has a high
toxicity which precludes its systemic use. It is present in many topical
antibiotic preparations, and since it is not absorbed by the gut, it is
given to «sterilize» the bowel prior to surgery.

2. Cell membrane inhibitors disorganize the structure or inhibit the function
of bacterial membranes. The integrity of the cytoplasmic and outer
membranes is vital to bacteria, and compounds that disorganize the
membranes rapidly kill the cells. However, due to the similarities in
phospholipids in eubacterial and eukaryotic membranes, this action is
rarely specific enough to permit these compounds to be used
systemically. The only antibacterial antibiotic of clinical importance
that acts by this mechanism is polymyxin, produced by Bacillus
polymyxis. Polymyxin is effective mainly against Gram-negative bacteria
and is usually limited to topical usage. Polymyxins bind to membrane
phospholipids and thereby interfere with membrane function.
Polymyxin is occasionally given for urinary tract infections caused by
Pseudomonas aeruginosa that are gentamicin, carbenicillin and
tobramycin resistant. The balance between effectiveness and damage
to the kidney and other organs is dangerously close, and the drug should
only be given under close supervision in the hospital.

3. Protein synthesis inhibitors Many therapeutically useful antibiotics owe
their action to inhibition of some step in the complex process of
translation. Their attack is always at one of the events occurring on the
ribosome and rather than the stage of amino acid activation or
attachment to a particular tRNA. Most have an affinity or specificity
for 70S (as opposed to 80S) ribosomes, and they achieve their selective
toxicity in this manner. The most important antibiotics. with this mode
of action are the tetracyclines, chloramphenicol, the macrolides (e.g.
erythromycin) and the aminoglycosides (e.g. streptomycin).

The aminoglycosides are products of Streptomyces species and are
represented by streptomycin, kanamycin, amikacin and gentamicin.
These antibiotics exert their activity by binding to bacterial ribosomes
and preventing the initiation of protein synthesis. Aminoglycosides have
been used against a wide variety of bacterial infections caused by Gram-
positive and Gram-negative bacteria. Streptomycin has been used
extensively as a primary drug in the treatment of tuberculosis.
Gentamicin is active against many strains of Gram-positive and Gram-
negative bacteria, including some strains of Pseudomonas aeruginosa.
Kanamycin (a complex of three antibiotics, A, B and C) is active at low                   concentrations against many Gram-positive bacteria, including
penicillin-resistant staphylococci. Gentamicin and amikacin are
mainstays for treatment of pseudomonas infections. An unfortunate
side effect of aminoglycosides has tended to restrict their usage:
prolonged use is known to impair kidney function and cause damage
to the auditory nerves leading to deafness.

The tetracyclines consist of eight related antibiotics which are all natural
products of Streptomyces, although some can now be produced
semisynthetically. Tetracycline, chlortetracycline and doxycycline are the
best known. The tetracyclines are broad-spectrum antibiotics with a wide
range of activity against both Gram-positive and Gram-negative bacteria.
The tetracyclines act by blocking the binding of aminoacyl tRNA to the
A site on the ribosome. Tetracyclines inhibit protein synthesis on isolated
70S or 80S (eukaryotic) ribosomes, and in both cases, their effect is on
the small ribosomal subunit. However, most bacteria possess an active
transport system for tetracycline that will allow intracellular accumulation
of the antibiotic at concentrations 50 times as great as that in the medium.
This greatly enhances its antibacterial effectiveness and accounts for its
specificity of action, since an effective concentration cannot be
accumulated in animal cells. Thus a blood level of tetracycline which is
harmless to animal tissues can halt protein synthesis in invading bacteria.
The tetracyclines have a remarkably low toxicity and minimal side
effects when taken by animals. The combination of their broad spectrum
and low toxicity has led to their overuse and misuse by the medical
community and the widespread development of resistance has reduced
their effectiveness. Nonetheless, tetracyclines still have some important
uses, such as in the treatment of Lyme disease.

Chloramphenicolhas a broad spectrum of activity but it exerts a
bacteriostatic effect. It is effective against intracellular parasites such
as the rickettsia. Unfortunately, aplastic anemia, which is dose related
develops in a small proportion (1/50,000) of patients. Chloramphenicol
was originally discovered and purified from the fermentation of a
Streptomyces, but currently it is produced entirely by chemical synthesis.
Chloramphenicol inhibits the bacterial enzyme peptidyl transferase
thereby preventing the growth of the polypeptide chain during protein
synthesis. Chloramphenicol is entirely selective for 70S ribosomes and
does not affect 80S ribosomes. Its unfortunate toxicity towards the small
proportion of patients who receive it is in no way related to its effect on
bacterial protein synthesis. However, since mitochondria probably
originated from prokaiyotic cells and have 70S ribosomes, they are
subject to inhibition by some of the protein synthesis inhibitors
including chloramphenicol. This likely explains the toxicity of
chloramphenicol. The eukaryotic cells most likely to be inhibited by chloramphenicolare those undergoing rapid multiplication, thereby
rapidly synthesizing mitochondria. Such cells include the blood forming
cells of the bone marrow, the inhibition of which could present as
aplastic anemia. Chloramphenicol was once a highly prescribed
antibiotic and a number of deaths from anemia occurred before its use
was curtailed. Now it is seldom used in human medicine except in life-
threatening situations (e.g. typhoid fever).

The macrolides are a family of antibiotics whose structures contain large
lactone rings linked through glycoside bonds with amino sugars. The
most important members of the group are erythromycin, azithromycin
and clarithromycin. Erythromycin is active against most Gram-positive
bacteria, Neisseria and Legionella, but not against the Enterobacieriaceae
and Haemophilus. Macrolides inhibit bacterial protein synthesis by
binding to the 50S ribosomal subunit. Binding inhibits elongation of
the protein by peptidyl transferase or prevents translocation of the
ribosome or both. Macrolides are bacteriostatic for most bacteria but
are cidal for a few Gram-positive bacteria.

4. Effects on Nucleic Acids Some chemotherapeutic agents affect the
synthesis of DNA or RNA, or can bind to DNA or RNA so that their
messages cannot be read. Either case, of course, can block the growth
of cells. The majorities of these drugs are unselective, however, and
affect animal cells and bacterial cells alike and therefore have no
therapeutic application. Two nucleic acid synthesis inhibitors which
have selective activity against procaryotes and some medical utility are
nalidixic acid and rifamycins.

Nalidixic acid is a synthetic chemotherapeutic agent which has activity
mainly against Gram-negative bacteria. Nalidixic acid belongs to a
group of compounds called quinolones. Nalidixic acid is a bactericidal
agent that binds to the DNA gyrase enzyme (topoisomerase) which is
essential for DNA replication and allows supercoils to be relaxed and
reformed. Binding of the drug inhibits DNA gyrase activity.

Some quinolones penetrate macrophages and neutrophils better than
most antibiotics and are thus useful in treatment of infections caused by
intracellular parasites. However, the main use of nalidixic acid is in
treatment of lower urinary tract infections (UTI). The compound is
unusual in that it is effective against several types of Gram-negative
bacteria such as Escherichia coli, Enterobacter aerogenes, Klebsiella
pneumoniae and Proteus spp. which are common causes of UTIs. It is
not usually effective against P. aeruginosa, and Gram-positive bacteria
are resistant. Newer compounds are characterized by wider spectrum of
activity. For example, ciprofloxacin has an excellent primary activity
against P. aeruginosa, levofloxacin and moxifloxacin both have very good
activity against gram-positives (especially Streptococcus pneumoniae).

The rifamycins are also the products of Streptomyces. Rifampicin is a
semisynthetic derivative of rifamycin that is active against Gram-positive
bacteria (including Mycobacterium tuberculosis) and some Gram-negative
bacteria. Rifampicin acts quite specifically on eubacterial RNA polymerase
and is inactive towards RNA polymerase from animal cells or towards DNA
polymerase. The antibiotic binds to the beta subunit of the polymerase
and apparently blocks the entry of the first nucleotide which is necessary
to activate the polymerase, thereby blocking mRNA synthesis. It has been
found to have greater bactericidal effect against M. tuberculosis than other
anti-tuberculosis drags, and it has largely replaced isoniazid as one of the
front-line drugs used to treat the disease, especially when isoniazid
resistance is indicated. It is effective orally and penetrates well into the
cerebrospinal fluid and is therefore useful for treatment of tuberculosis
meningitis and meningitis caused by Neisseria meningitidis.

5. Competitive Inhibitors The competitive inhibitors are mostly all
synthetic chemotherapeutic agents. Most are «growth factor analogs»
which are structurally similar to a bacterial growth factor but which do
not fulfill its metabolic function in the cell. Some are bacteriostatic
and some are bactericidal.

Sulfonamides were introduced as chemotherapeutic agents by Domagk
in 1935, who showed that one of these compounds (prontosil) had the
effect of curing mice with infections caused by beta-hemolytic
streptococci. Chemical modifications of the compound sulfanilamide
gave compounds with even higher and broader antibacterial activity.
The resulting sulfonamides have broadly similar antibacterial activity,
but differ widely in their pharmacological actions. However, due to
severe side effects, their clinical significance currently is very limited.
The sulfonamides and trimethoprim are inhibitors of the bacterial enzymes
required for the synthesis of tetrahydrofolic acid (THF), the vitamin form
of folic acid essential for 1 -carbon transfer reactions. Sulfonamides are
structurally similar to para aminobenzoic acid (PABA), the substrate for
the first enzyme in the THF pathway, and they competitively inhibit that
step. Trimethoprim is structurally similar to dihydrofolate (DHF) and
competitively inhibits the second step in THF synthesis mediated by the
DHF reductase. Animal cells do not synthesize their own folic acid but
obtain it in a preformed fashion as a vitamin. Since animals do not make
folic acid, they are not affected by these drugs, which achieve their
selective toxicity for bacteria on this basis.

Three additional synthetic chemotherapeutic agents have been used in the
treatment of tuberculosis: isoniazid (INH), paraaminosalicylic acid (PAS), and
ethambutol. The usual strategy in the treatment of tuberculosis has been to administer
a single antibiotic (historically streptomycin, but now, most commonly, rifampicin
is given) in conjunction with INH and ethambutol. Since the tubercle bacillus rapidly develops resistance to the antibiotic, ethambutol and INH are given to prevent
outgrowth of a resistant strain. It must also be pointed out that the tubercle bacillus
rapidly develops resistance to ethambutol and INH if either drug is used alone.
Ethambutol inhibits incorporation of mycolic acids into the mycobacterial cell wall.
Isoniazid has been reported to inhibit mycolic acid synthesis in mycobacteria and
since it is an analog of pyridoxine (vitamin B₆) it may inhibit pyridoxine catalyzed
reactions as well. Isoniazid is activated by a mycobacterial peroxidase enzyme and
destroys several targets in the cell. PAS is an anti-folate. PAS was once a primary
anti-tuberculosis drug, but now it is a secondary agent.

Side effects of antibiotics and preventive measures.

E) disturbance of the gastrointestinal tract (tetracyclines, antitumor antibiotics).

Rare, but more serious side effects, include the formation of kidney stones with the sulphonamides, abnormal blood clotting with some of the cephalosporins, increased sensitivity to the sun with the tetracyclines, blood disorders with trimethoprim, and deafness with erythromycin and the aminoglycosides. Sometimes, particularly in older people, antibiotic treatment can cause a type of colitis (inflamed bowel) leading to severe diarrhoea.

2. Dysbiosis occurs with long-term treatment with broad-spectrum antibiotics. Not only pathogenic microbes, but also representatives of normal microflora, are killed. The place for antibiotic-resistant microbes is released, which can cause various diseases. The most common diseases that occur during dysbiosis are diarrhea, fungal infections of the mouth, digestive tract and vagina (сandidiasis) because antibiotics destroy the protective 'good' bacteria in the body (which help prevent overgrowth of any one organism), as well as the 'bad' ones, responsible for the infection being treated.

To prevent dysbiosis, antibiotics of a narrow spectrum of action should be used, antibiotics should be combined with antifungal drugs to destroy fungi and with probiotics to restore normal microflora.

3. Negative effects on the immune system:

A) development of allergic reactions (10% of cases); the most powerful allergens are penicillins, cephalosporins; 'allergic' reactions to antibiotics such as penicillin have been documented in medical literature for over forty years. The severity of these side-effects may range from a simple rash to anaphylaxis (swelling of the face and tongue), a life-threatening reaction which includes difficult or labored breathing, which are also symptoms of an asthmatic attack. To prevent allergic reactions you need to know the individual sensitivity of people, to prevent anaphylactic shock - do skin-allergic tests.

B) suppression of immunity (immunosuppression): levomycetin inhibits the formation of antibodies, cyclosporin A - the function of T-lymphocytes; For prevention - a strict approach to prescribing antibiotics.

C) disturbance of the formation of a full-fledged immunity after the transfer of an infectious disease; this is due to the insufficient antigenic action of microbes that die from antibiotics before they can perform the antigenic function; as a result, there are repeated infections (reinfection) occur; for the prevention it is necessary to combine antibiotics with the vaccine (antibiotics cause death of pathogens, and the vaccine forms immunity).

4. Reaction exacerbation - the development of intoxication as a result of the isolation of endotoxins from microbial cells in the event of their mass death (cell destruction) under the action of antibiotics.

5. The negative effect of antibiotics on fetal development. This occurs as a result of damage to the mother's body, spermatozoa, placenta and metabolic disorders of the fetus. For example, tetracycline exerts a direct toxic effect on the fetus. There are cases of the appearance of children-freaks. In 1961, because of the use of thalidomide, children were crippled without hands, without legs and there were cases of child mortality.

In order not to cause harm to the human body, antibiotics should have a specific tropicity, i.e. their action should be purposeful to suppress (destroy) pathogenic microbes. Antibiotics should also have organotropicproperties - the antibiotic's property to selectively affect certain organs. For example, enteroseptol is used to treat intestinal infections. It is practically not absorbed from the digestive tract.