microbiology, lecture 11

8/7/2019 Microbiology, Lecture 11

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Before starting , I wanted to notify some points, this lecture was the

lecture we had after the anatomy first exam , it was a real mess .. We werea bit noisy, the record was not that clear... The lecture starts at slide

number 42.. The record we got starts 3 slides later...Slide 45 I've justcopied the slides for the first missing three and for the other parts which

were not clear I've added some notes from our textbook. That was the bestwe could do ,sorry for any mistake. The S.R

Synergism Versus Antagonism:

Synergism is when 2 antimicrobial agents are used together to produce adegree of pathogen killing that is greater than that achieved by either drugalone. Synergism is a good thing!

Antagonism is when 2 drugs actually work against each other. Theextent of pathogen killing is less than that achieved by either drug alone.Antagonism is a bad thing!

Antimicrobial Combinations Indications for Use

_ Prevention of the emergence of resistant organisms_ Polymicrobial infections_ Initial therapy_ Decreased toxicity_ Synergism

Now the 2 concept that we need to understand whenever we talk aboutantimicrobial resistance:

1. The MIC: minimal inhibitory concentration

The MIC is the lowest conc. Of any microbial agent require to inhibit thegrowth of a microbe.In other words :the lowest concentration of theantimicrobial agent required to achieve a bacteriostatic effect.

2. The MBC: minimal bactericidal concentration

The MBC the lowest conc. Of antimicrobial agent require to kills 99.9%of the population, ex, if you have 1000 bacterial cells and you add a

certain conc. Of drug at the MBC, only 1 cell will survive. So MBCdoesn't mean kill all organisms, just 99.9% of them or more.

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We can see that a certain organism is susceptible to a certain drug, if youcan give the patient a certain dose of anti microbial agent that can be toachieving the MIC at the certain infected tissue, because we cannot givethe patient unlimited amount of microbial agent, just only minimal conc.

Other while you cannot achieve the MIC conc. Of a tissue of the patientthat you considered that the resistant to the drug.

Susceptible: The MIC of the organism can be achievedin the body at recommended doses.

Resistant: The MIC cannot be achieved.

Multiple ways that you can measure the MIC & MBC for a particulardrugs toward a particular pathogen, regard to MIC there is the (diskdiffusion test & dilution test), you can skip this because you will learn it

in the lab. Regarding the MBC there's also another way we usually gothrough a tube dilution method.

Because you are going to learn all these in the lab. we will skip them inthe theory.

As you heard in lectures and news there are many microorganisms thatthere are resistant to anti microbial agents.So the major problems inmedicine regarding antimicrobial agent resistant:

a strain of Staphylococcus aureus called Methicillin Resistant(MRSA)

Vancomycin resistant enterococci (VRE) andstaphylococci(Vancomycin resistant)

also Penicillin resistant Streptococcus pneumonia,causing bacterialpneumonia in humans .now we are having more and more stains ofPenicillin resistant.

Multiple antibiotic resistant Gram-negative bacilli ,localized with

various infections.

Multiple antibiotic resistant Mycobacterium tuberculosis; Multi-Drug Resistant (MDR-TB); Extensively Drug-Resistant (XDR-TB).

Regarding the method of resistance, some organism are naturallyresistant to a certain drugs because:

1-They lack the target of the drug.

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2-some drug require entry to the cell,so some drug cannot naturally enterthe cell membrane or the cell wall. So thats what's called intrinsicresistance or natural resistance. example of this sederemones eregenosa;its membrane have very weak permeability towards a lot of drugs that can

fit another organisms.In contrast to intrinsic resistance some organism acquire resistance todrug that used to be active against it, example streptococcus pneumoniathat treat with penicillin but now they acquire a various gene or mutationthat make it resistant to penicillin. We have multiple ways to acquireresistance:

> anti microbial agent before enter the cell need to bind a certain cellmembrane receptor ,so a mutation to the cell membrane receptor or

the drug binding side ,will make the drug not able to bind and themicrobe become resistant to the drug, sometimes the cell membraneitself will encounter some mutation and will not allow the drug to

pass through the cell wall,

> Also some organism use enzyme or acquire enzyme that caninactivate the drug, example B-Lactamases.

Usually the way bacteria acquire resistant gene is through manlyconjugation, if you remember when we talked about conjugation ,we said

a plasmid which has multiple genes that transfer through conjugation iscalled resistant factor or R factor.

> Another mechanism in which bacteria can be resistant to multipledrugs at the same time is by receiving a gene that encode for aMDR pump (multidrug resistance), so this pump extent on the cellmembrane and take all the antibiotic that enter the cytoplasm and

pump it outside the cell .

The table summarizes the various methods (table 9-7(

That was about the general mechanism now will talk about specificmethod regarding each of the important group of the anti microbialagent.

For penicillin and cephalosporin which are both B-lactam drug, manybacteria are able to escape from the Pencillin drug by producing -lactamese enzyme which can hydrolyze the B-lactam ring; for the drugsthat are B-lactamase resistant, the enzyme itself (that target the B-lactamring )can undergo mutation that don't allow the Penicillin or

cephalosporin to bind.

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For tetracycline we mentioned last time that many bacteria actively pumptetracycline inside the cell so one way to become resistance to tetracyclineis to pump the tetracycline out the cell.

For Chloramphenicol and Aminoglycosides some bacteria basicallyencode enzymes that chemically modify the 2 drugs to make themchemically inactive.

For Sulfonamides & Trimethoprim, inhibit two enzymes in thetetrahydrofolic acid pathway, bacteria become resistance by havingbackup copies of the two enzymes that are not affected by the drug.

Finally Erythromycin & Lincomycin , target is rRNA in the 50S unit ofthe ribosome, so one way that bacteria become resistance to the drug byundergoing enzymatic modification (methylation) of 23S rRNA that willnot allowed the drug to bind it.

So here are the two B-lactamasese ,the penicillase which inactivate the B-lactam ring in penicillin and penicillin derivatives. And thecephalosporinase which hydrolyses the B-lactam ring of thecephalosporin .So this is one way bacteria can become resistant toPencillin and cephalosporin ;and the second way I mentioned is that thetranspeptidases,which are diluted drugs undergo mutation that doesn'tallow this drug to bind.,

So this are some examples of aminoglycoside, and these basically canmetabolic by various enzymes of the bacteria so these becomePhosphorylated, Adenylated and Acetylated and thus making these drugsinactive.

And this is how some bacteria become resistant to Sulfonamides &Trimethoprim ,so these 2 enzymes, synthetase & dihydrofolate reductase,are target by these 2 drugs, so the bacteria that's resistant to these drugsreplaces these enzymes with other versions that are not resistant to the

drug.This is tetracycline resistance, so this enzyme is actively pumped insidethe cell, and the resistance organism acquire a gene for the efflux pump.

Some Strategies in the War Against Drug Resistance

(The doctor just read the slide .. Without adding a word )

> Education of healthcare professionals and patients>

Patients should stop demanding antibiotics every time they are, ortheir child is, sick

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> Physicians should not be pressured by patients and should prescribedrugs only when warranted

> Clinicians should prescribe a narrow-spectrum drug if lab resultsindicate that it kills the pathogen( don't need a broad spectrum,

don't need to kill all organism if you know the pathogen)> Patients should destroy any excess or out-dated medications> Antibiotics should not be used in a prophylactic manner( as if you

feel like .. an infection is to occur, you shouldn't take anantimicrobial agent in order to prevent this infection, even beforehappening )

> Healthcare professionals should practice good infection control> Patients should take drugs in manner prescribed> Elimination of antimicrobial agents from animal feeds

So they key words arePrevent Infection; Diagnose & Treat Effectively;Use Antimicrobials Wisely.

Empiric therapy

Sometime if we had medical emergence the physician can't wait for labresults ,so if the physician knows that there's an infection and highlysuspects that E.coli is the cause of this infection he can start to give the

patient the drug that he knows is effective with E.coli . so basically the

physician make an "educated guess" based on past experience with thetype of infection disease and the most effective drugs. And there are manyfactors that determine what drug we have to give the patient in the empirictherapy.

> One important aspect is called pocket chart, (the doctor read someof the chart's numbers you can find it in the slide and read it all if uwant). Pocket chart For aerobic Gram-negative bacteria. The chartis a quick reference whenever empiric therapy is necessary. So

based on this chart if you get a patient with medical emergencewith resistant E.coli you can give the patient one of these affectivedrugs listed in the orange arrow in the table (vertically).

> You have to know if the patient allergic to any drugs, many peopleallergic to penicillin and you can't give them Penicillin becausethey might have prophylactic shock (I'm not sure about this) andmight die even.

> The age of the patient is very important, not all drugs can be givento all age's groups.

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> If the patient is pregnant ,some drugs are known as teratogenic,meaning that they can lead to growth abnormalities .

> Whether the patient is Inpatient or outpatient, whether need to stayin hospital or can go home; if he's an inpatient (needs to go inside,hospital or clinic) you can give him some drugs that can't be givenoutpatients; some drugs should be administrated intravenously andtherefore cant be prescribed to outpatients which can be givenoptical or oral agents.

> Site of infection is very important; not all drugs are active to allbody tissues, or some might concentrate in other tissue more than inothers.

> Some antimicrobial; agent might have cross reaction with othermicrobial agents that can be harmful to the patient.

> If the patient has any medical problems.

> If the patient is leukopenic or immunocompromised; patient withcompromised immune system should not be given bacteristaticagents, should only be given bacteriocidial agents.

> The cost of the drug.

Undesirable Effects of Antimicrobial Agents:

One side effect (reason, outcome) of not using antimicrobial agentsproperly is that you will end up with new strains that are resistant toresistance drug; this is known as selecting for resistant organisms.

If it's given (the antimicrobial agent) randomly, not carefully, thepatient may become allergic to the agent.

Many agents are toxic to humans and some are very toxic. With prolonged use,(at each infection,) a broad-spectrum antibiotic

may destroy the normal flora, resulting in an overgrowth of bacteriaknown as a super infection .The person has no longer indigenousmicro flora and thus becoming more susceptible to infectionscaused by opportunistic invaders.

Slide 68 : Selecting for drug-resistant organisms

This is located in your body ,when you have an infection :most of theorganisms in that site will be sensitive to the antimicrobial agent that is to

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be given ,and a few will have already a developed mutation that makethem resistant to the drug the continuation was not clear ,I'll copy thereferred part from the textbook:The first picture is indigenous micro flora of a patient before initiation of

antibiotic therapy. Most members are susceptible (sensitive) to theantibiotic to be administrated; very few are resistant.The second picture: after antibiotic therapy has been initiated, thesusceptible organisms are dead, only few resistant organisms remain;The third picture: as a result of decreased competition for nutrients andspace, the resistant organisms multiply and become predominantorganisms in the patient's indigenous micro flora.

You all know that we have human infection with protozoa and fungi andalso with viruses. Antiprotozoal Agents: Antiprotozoal agents are usually toxic to

the host.They work mainly by :

- Interfering with DNA and Rna synthesis- And by interfering with protozoal metabolism Antifungal Agents: Most antifungal agents work in one of 3ways:

By binding with cell membrane sterols By interfering with sterol synthesis By blocking mitosis or nucleic acid synthesis

We have a problem with antiprotozoal and antifungal agents ,these twoorganisms are eukaryotes ,so many of the targets ,to some extents, aresomewhat similar to human pathogens ,so human structures and humanenzymes. So they are associated with higher toxicity than antimicrobialagents.

Antiviral agents are the newest agents to develop because don'tundergo (don't have) any active metabolism. They use up host cells

to replicate themselves and some drugs have been developed thatare effective in certain viral infections, but not others; they work byinhibiting viral replication within cells; (you can't targetantimicrobial cell and make an antiviral agent)

In some cases you have to give antiviral agent mixed, several drugs thatare administered at the same time ,together, are being used, as to treatHIV infection.

Done By : Yazeed Al-Ajlooni and Sara Ibdiwi

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microbiology, lecture 11

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