م

number 26

Done by َمها أبو عجميّة

Corrected by Lara Abdallat

Doctor Dr. Malik

You should be familiar with the following points: (they were mentioned in

the lecture)

- Empirical Therapies : based on experience and, more specifically,

therapy begun on the basis of a clinical educated guess in the

absence of complete or perfect information. (it is given blindly/ no

cultures are sent to lab)

Definitive Therapies: The treatment plan for a disease that

has been chosen as the best one for a patient based on

tests made, in other words, you know exactly the cause of

infection after getting results from the cultures you’ve sent

(or by sensitivity test) and then decide to choose the best

antibiotic.

- Bactericidal (kills bacteria) and Bacteriostatic(inhibits bacteria and

prevents bacterial growth )agents

- Bactericidal agents should be selected over bacteriostatic ones for

treatment of critical conditions and infections (such as

endocarditis, meningitis, septicemia, sepsis, bacteremia -don’t

memorize them-), where the bacteria can be found all over the

body (systemically). In other words, in hospitals, we don’t usually

give statics, we usually use the cidals.

- MIC (minimum inhibitory concentration) of a certain drug depends

on the microorganism type, because each drug has an MIC that is

different from a bacteria to another)for example we cant say that the

MIC of penicillin is 5! Because we didn’t determine on what kind of

bacteria the MIC is 5, is it for staph?strep?, this is why when you

read an antibiotic’s leaflet, you will see that the drug is given at

different doses, depending on the type of the infection/ bacteria.

- Bacterial gyrases are like humans’ topoisomerases which help

relieve (the tight twisting/ the strain/ supercoiling) which is formed

during replication by breaking and rejoining DNA strands,

polymerases catalyze the synthesis of DNA by adding nucleotides

to the (preexisting) chain.

- Knowing the antibiotics’ names is not enough, we should know their

spectrum of activity – what bacteria is the antibiotic effective

against?-

- One more added microorganism to spectrum increases the

antibiotic’s likelihood to be indicated (esp. for empirical treatment)

- The major problem threatening the continued use of antibiotics is

the development of resistant bacteria. But that doesn’t mean that all

bacteria are resistant, sensitive bacteria are still present and can

cause infection

o (e.g: penicillin G is still the antibiotic of choice (empirically),

for patients that are diagnosed with strep throat which is

caused by Streptococcus, streptococcus has not even

developed beta lactamase activity yet!)

- This slide is our reference in this course, it is of great importance,

we know that there are many antibiotics and many brands; but which

one to choose?

Answer: based on a series of rational steps,we took basic idea about

this in the previous lectures, we choose antibiotics according to site

of infection, source of infection and the narrowest possible spectrum

(the narrowest is the best as the doctor says)

This slide is giving details about the site of infection

Main molecular targets for antibiacterial chemotherapy (antibiotics):

- External Integrity of the bacterial Cell Wall (Cell wall synthesis),

They are generally bactericidal.

They are the oldest clinically used, All β-lactam compounds, so

named because of their four-membered beta lactam ring:

penicillin, cephalosporins, monobactams, carbapenems,

vancomycin (and his brother teicoplanin)

In this sheet we will have some details about penicillin G,

Benzathine Penicillin, Penicillin V, Aminopenicillins and beta

lactamase inhibitors.

- Protein synthesis

Such as tetracyclines, macrolides, chloramphenicol, clindamycin

and aminoglycosides, they are all bacteriostatic except

aminoglycosides, aminoglycosides are cidals.

bacteriostatic agents put the bacterial cells in a static state

(permanent growth arrest) without undergoing cell death, this

happens to our cells, too(senescence)

1) Why Aminoglycosides are bactericidal?

They have a dual activity/ multiple mechanisms of action (in addition

to interfering with protein synthesis, they can also affect the integrity

of bacteria and the fluidity of its membrane) But the precise

mechanism for bactericidal activity is not known. This possible

duality or the multiple mechanisms of action, might be true since

there is no real resistance against aminoglycosides, (about 97% of

E.coli are still sensitive to gentamycin), aminoglycosides are still

used (but not orally)

- Perturbation of nucleic acid synthesis

They are called antimetabolites, There are some bacteriostatic and

some bactericidal antibiotics in this group, here are some

mechanisms,

1) In bacteria, folic acid synthesis pathway is really important

for synthesizing bacterial nucleic acids, we have two enzymes

(a reductase and a synthase) in this pathway, their inhibition

can lead to inhibiting the synthesis of pyrimidines and purines

(building blocks of nucleic acid) by either Sulphonamides or

Trimethoprim

Note: the names of enzymes are not important.

2) Another mechanism to prevent the synthesis of nucleic

acids is by giving analogues of pyrimidines and purines

(molecules that resemble them), then the polymerase add

them to the nucleic acid chain

3) Inhibitors of gyrases/ topoisomerase IV (such as quinolones,

they are really clinically great, resistance to them is difficult)

while DNA is being continuously synthesized, coiling/ strains

occur, here comes the gyrase to break and rejoin to enable the

polymerase to continue its job, what quinolones do? When

Gyrases start working by cleaving the supercoiling , the

antibiotics (quinolones) trap it ,in the moment of cleaving the

chain, in a complex , the chain remains cleaved, with no enzyme

to rejoin it!

This gyrase is essential for bacterial cell, quinolones don’t

actually inhibit the gyrase itself, but trap it, so in a way the gyrase

is the one which caused the bacterial death-poisining-

(quinolones are strong bactericidal)

4) inhibitors of RNA polymerases, in respiratory system, such

as anti-TB rifampicin,

Rifampicin has a cidal activity, the strongest *inducer* of

cytochrome p450 .(it is a high yield question)

Cell wall inhibitors

- These agents interfere with synthesis of the bacterial cell wall

(mammalians cells do not have it).

“generally speaking” these inhibit transpeptidases (remember from

microbiology: they are also known as, PBP -penicillin binding

proteins), they work by preventing the linkages (cross-linking) in the

cell wall of bacteria when they are multiplying and synthesizing their

cell wall, which will lead to leakage of the bacterial contents, and

disruption of the osmotic pressure difference, thus these cell wall

inhibitors are -bactericidal antibacterial agents-

((Vancomycin has a different mechanism of action, go to

microbiology lectures if you’d like to remember, but generally It has

a unique mode of action inhibiting the second stage of cell wall

synthesis of susceptible bacteria..))

- To be maximally effective, cell wall inhibitors require actively

proliferating (multiplying) bacteria. As previously mentioned, these

antibiotics cannot target already-synthesized cell wall, but they

target the bacteria when they are dividing, when their cell walls are

being synthesized, it is when transpeptidases are working on the

two bacteria’s cell walls after division.

- A general Rule: although the combination of bactericidal with

bacteriostatic is a theory (which is found in almost every book), we

don’t usually use it practically because if the cell wall was linked,

there would be no use of the bactericidal, because if you gave your

patient a bacteriostatic drug followed by a bactericidal drug, the

antibiotics’activity would decrease as a result of drug-drug

interaction. Think about it! What is the use of stopping the replication

(no mutliplying), and then giving a bactericidal drug, the cell wall is

already synthesized(no use!) this rule is only theoretical BUT it can

be used in some cases, for instance in peptic ulcer, we might use

erythromycin (static) and amoxicillin (cidal),

The main forms of Penicillins resistance

Resistance to penicillins and other β-lactams is due to one of these

general mechanisms:

1. β-lactamases (penicillinases) which hydrolyse the lactam ring.

β-lactamase production is particularly important in staphylococci,

but they are not produced by streptococci. (streptococcus is

sensitive)

At least 90% of staphylococcus species in the West now

produce β -lactamases.

To overcome the problem, β -lactamase antagonists such as

clavulanic acid which is a suicide inhibitor of the enzyme, are

used.

2. reduction in the permeability of the outer-membrane of Gram-

negative bacteria (remember: this outer membrane is not present in

gram- positive bacteria)

3. mutations to the penicillin-binding proteins (transpeptidases).

- ExtraNote: referring to point 2, it is true that gram-negative

resistance can develop due to the impermeable outer membrane of

their cell, but in this sheet, we will see that Beta-lactam antibiotics

can cross the outer membrane and enter gram-negative organisms,

but how? via outer membrane protein channels called porins.

Penicillins:

1) The Natural Penicillin (penicillin G/ benzylpenicillin)

Naturally-occurring benzylpenicillin (penicillin G) is active

against some organisms but their main drawbacks are

o penicillin G is sensitive to acid hydrolysis in the

stomach, which means it has to be administered by

injection, but not orally administered because it is unstable

in the stomach.

o Its susceptibility to β -lactamases. (penicillinase-

sensitivity)

o Limited activity against gram negatives.

These, and other, drawbacks gave inspiration to the

development of the semi-synthetic agents designed to be :

1) b-lactamase resistant

2) broad spectrum

3) extended spectrum (extended to include pseudomonas) –

the following lecture

The natural Benzylpenicillin (Penicillin G) is Given IM or IV.

Active against (its spectrum):

most gram-positive bacteria (mainly streptococcus) with the

exception of penicillinase-producing Staphylococcus aureus

( but active on staphylococcus without this -ase activity, which

is not present in life nowadays)

limited activity on gram negative bacteria, it is only active on

some gram-negative anaerobes and most Neisseria species

(such as Neisseria gonorrhoeae and Neisseria meningitidis)

not active against most gram-negativeaerobic organisms

Realistically-speaking, it is active on Neisseria and Streptococci,

it has no actual staph. activity, it is not our drug of choice in

patients with staph infections (because sensitive Staph are no

longer existent اللي استحوا ماتوا)

clinical uses of penicillin G

- Empirical Treatment for beta–hemolytic streptococcal pharyngitis

(aka, Strep Throat).

This Strep Throat (pharyngitis), is characterized by the

presence of white spots on the surface of the throat, why

we can empirically use penicillin g? Because

Streptococcus is the only cause of this strep throat, and

penicillin G is active against streptococci.

- Refer to the important slide (page 2) and see how penicillin G

cannot be used empirically for patients with upper respiratory tract,

urinary tract and ALL the other infections, we conclude that this

penicillin G can only be used empirically to treat people with strep

throat.

- Streptococcal infections that include pneumonia, otitis media,

meningitis, and septic arthritis. Penicillin G is used only as

definitive therapy.

o When a patient is diagnosed with meningitis, we don’t use

penicillin G empirically because it is true that Neisseria

meningititidis and Sterptococci can be possible causes, but

there are many other possible bacteria can cause

meningitis which cannot be targeted by penicillin G, such

as E. coli (refer to the slide in the second page) penicillin G

is then effective against Neisseria meningitis (definitive for

N.meningitis), meaning that if you are sure that Neisseria

meningitidis is the cause of meningitis, you are allowed to

use penicillin G

o Another example is pneumonia, it is known that

Streptococcus pneumoniae is the most common cause of

it, but we cannot use it empirically because there are many

other bacteria can cause it too

- Penicillin G can be active on single microorganisms such as

Clostridium tetani, and Corynebacterium diphtheriae, Treponema

pallidum (syphilis), and Listeria monocytogenes. (in systems, we

will have more details)

A student’s question

Q: can we use penicillin in the previous cases empirically?

A: each one of the previous cases (Diptheria, Tetanus, Syphillis..)

is actually caused by one known microorganism, but anyway, they

are used as 1st line drugs.

- The autoimmune Rheumatic fever follows streptococcal infections,

(so it might be a complication of strep throat), if patients were

diagnosed with streptococcal infections more than once (repeatedly

infected), the patient’s condition might worsen, what is the solution?

It is to prevent streptococcal infection by giving the patients penicillin

g prophylactically (prophylaxis: treatment given to prevent disease.)

daily ,by injecting it from the day of diagnosis, so if a 6-year-old child

was diagnosed, he will be given prophylactic doses till 21, it is used

as streptococcal prophylaxis and to prevent the recurrence of

rheumatic fever .

But due to the difficulty of giving a patient an injection of penicillin G

daily for this long time, we found benzathine penicillin so we would

be able to give this patient an IM injection once a month instead, as

if it makes a reservoir , which takes 2-3 weeks for a sustained

release, as discussed more below.

-

Clinically, penicillin g is usually used in prophylaxis (rarely in

treatment)

2) Benzathine penicillin

Benzathine penicillin for intramuscular injection (IM) yield low but

prolonged drug levels (injected intramuscularly because it is

formed by adding something to penicillin that makes it colloidal)

1) A single intramuscular injection of benzathine penicillin, 1.2

million units, is an effective treatment for β-hemolytic

streptococcal pharyngitis (strep throat)

Also prophylactic, given intramuscularly once every 3–4

weeks, it prevents reinfection and recurrence of rheumatic

fever.

2) Benzathine penicillin G, 2.4 million units intramuscularly once

a week for 1–3 weeks, is effective in the treatment of syphilis

(compliance)

(DON’T memorize the number of units, you should only notice

that we doubled it in the case of syphilis, because each bacterium

has their own MIC -minimum inhibitory concentration- which is

different from other bacteria’s MIC, even for the same antibiotic.

Syphilis needs more to be killed.

3) The natural Penicillin (penicillin v/ phenoxymethylpenicillin)

Empirical treatment for beta–hemolytic streptococcal pharyngitis.

(strep throat)

It is more acid stable, it is orally active but is less potent

than penicillin G, more convenient to use it for children with strep

throat, because it can be taken orally, (instead of injections)

Adults with strep throat/ rheumatic fever, can use it if penicillin G

is not available,

It has the same spectrum as penicillin G but it is more active on

anaerobes, (Peptococcus, Peptostreptococcus, Actinomyces)

refer to the slide to check the names of bacteria

Penicillin V often employed in the treatment of oral infection,

where it is effective against some anaerobic organism.

Penicillin V is the most frequently prescribed antibiotic for oral

infections worldwide (because of its narrow spectrum) but we love to

be unique! :’)

It is the first choice in the treatment of odontogenic infections.

(1) post extraction infection, (2) pericoronitis and (3) salivary

gland infection.

Dentists use it.

4) B-lactamase-resistant Penicillins

These include Cloxacillin, Flucloxacilin, Oxacillin, methicillin.

methicillin is not available any more because of its side effects

and it was the one to blame for bacterial resistance (extra: methicillin

causes interstitial nephritis)

MRSA is very common. (MRSA = methicillin-resistant

Saphylococcus aureus)

Antibacterial spectrum is the same as for penicillin G, but

less potent and does cover staph producing beta- lactamase.

This group was synthesized because staphylococcus became

resistant to tetracyclineswhich were great targets to many gram

positive and gram negative bacteria.

Their use is restricted to treatment of infections caused by

penicillins-resistant bacteria. Nonetheless, Many

Staphylococci are now resistant to them.

They cover both Streptococcus and Staphylococcus, they are

good drugs to be used emipirically to skin,(also definitively),

Skin Infections -as the slide shows- are caused mainly by

Streptococcus pyogens, Staphylococcus aureus and

Staphylococcus epidermis … Hence, we don’t use penicillin v,

because it has no activity on staph

5) broad Spectrum Penicillins

Aminopenicillins

1. These include Ampicillin, which is fairly well absorbed orally (usually injections), Amoxicillin which is very well absorbed (usually orally), and is prodrug (a biologically inactive compound which can be

metabolized in the body to produce a drug) to ampicillin.

Their antibacterial spectrum is the same as for penicillin G

plus some Gram-negative bacteria. (aminopenicillins are made

by adding an amino group to the penicillin structure which makes

it more soluble in waterand enhances its ability to penetrate the

outer membrane of gram negatives by entering the porins)

Ampicillin and amoxicillin are among the most useful antibiotics

for treating children suffering from infections caused by

sensitive gram-negative aerobic bacteria,

enterococci,

β-lactamase-negative H.influenza

Aminopenicillins (Amoxicillin and Ampicillin) were developed to

increase activity against gram-negative aerobes

Gram Negative Gram Positive

Penicillin susceptible Staph aureus Proteus mirabilis

Penicillin susceptible Streptococcus Salmonella

Viridans Streptococci Some E.coli

Entercoccus sp. B-Lactamase-negativeH. influenzae

Listeria monocytogenes

6) B-Lactamase Inhibitor Combos

(Unasyn, Augmentin, Timentin, Zosyn)

Developed to gain or enhance activity against B-lactamase

producing organisms (some better than others). Provides some

or good activity against:

Anaerobes Bacteroides sp.

Gram-positive

S.aureus MSSA

Gram-negative

Neisseria gonorrheae

Moraxella catarrhalis

Klebseilla sp.

Proteus sp.

H. influenzae

E. coli

These can target those bacteria with beta lactamase activity such

as staph, H. influenza or even E.coli

Augmentin = Clavulanic Acid (a beta-lactamase inhibitor) +

Amoxicillin

Augmentin is the drug of choice to all upper respiratory tract

infections except strep throat.

We should thank Dr. Malik, our books and whoever invented Google and

Youtube, at the end of semester.