Mechanism of Antibiotic Resistance in Bacteria Flashcards Preview

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Flashcards in Mechanism of Antibiotic Resistance in Bacteria Deck (34)

Natural (intrinsic) resistance

-chromosomally mediated and is predictable


Mutational resistance

-random mutation
-secondary resistance occurring after therapy with the antimicrobial in question has begun


Transferable resistance

-is plasmid-mediated through:
-conjugation (bacterial mating)
-transduction (bacteriophage transmission)
-transformation (uptake of DNA from environment)


Induced resistance

-does not happen right away
-takes 20-30 days


Transposable Genetic Elements

-these are two types of transposable genetic elements: transposons, insertion sequences
-either element can translocate as an independent unit
-both elements are flanked on either end by short identical sequences of DNA in reverse order (inverted repeats)- these inverted repeat DNA termini are essential to the transposition process
-transposons and insertion sequences are incapable of autonomous self-replication and must exist on a replicon, such as the chromsome, bacteriophagem or plasmid to be replicated and maintained in a bacterial population


Insertion Sequences

-inserted sequences are mobile genetic elements that are known to encode only functions involved in insertion events
-IS may contain partial or complete promoters capable of activating the expression of neighboring genes
-allows expression of downstream genes presumably because the transposon is supplying a new promoter for those genes
-IS elements have proven to play an important role in the activation of resistant gene transcription in Staphylococci



-specialized sequences of DNA that possess their own recombination enzymes (transposases), allowing for transposition (hopping) from one location to another independent of the recombination enzymes of the host
-transposons can translocate as a unit from one area of the bacterial chromosome to another or between the chromosome and plasmid or bacteriphage DNA



-integrons are mobile DNA elements with the ability to capture genes, notabliy those encoding antibiotic resistance, by site-specific recombination
-the antibiotic resistance genes that integrons capture are located on gene cassettes
-the expression of promoterless, cassette-associated resistance genes is markedly influences by their position in a cassette array


Mechanisms of Antibiotic Resistance

-enzymatic inactivation
-decreased permeability
-alteration of target site
-protection of target site
-overproduction of target
-bypass of inhibited process



-bacterial enzymes that inactivate B-lactam antibiotics by hydrolysis of the B-lactamase bond
-they may differ in their substrate profiles, the drugs they can inactivate
-enzymes require either a serine or zinc atom moiety at the active site to hydrolyze the B-lactam ring


Decreased Susceptibility to the B-lactams

-Narrow spectrum B-lactamases- resistance to penicillins and narrow-spectrum cephalosporins

-Extended spectrum B- lectamases (ESBL's)- resistance to extended-spectrum cephalosporins (3rd generation) but remain susceptible to aztreonam

-ampC production- mostly chromosomal- plasmid mediated now being observed

-carbapenemase production (carbapenem-resistant enterobacteriaceae)

-major outer membrane protein modifications- decreased production of OmpC or OmpF may result in low level B-lectamase production


B-lactam inhibitor combinations

-currently availible products combine ampicillin, amoxicillin, ticarcillin, or piperaccillin with a beta lactamase inhibitor

-Clavulanate +amoxacillin = augmentin
-Clavulanate+ ticarcillin = timentin
-Sulbactam + amplicillin = Unasyn
-Tazobactam + piperacillin = Xosyn


Penicillinase Resistant Penicillins

-the drugs in this class are highly resistant to inactivation by staphylococcal penicillinase (B lactamase) and are active against B-lactamase producing Staphylococcus aureus
-Methicillin, Oxacillin, Cloxacillin, Dicloxacillin, Nafcillin


Extended Spectrum B- lactamases (ESBL's)

-this group of enzymes is emerging as a major problem in the US
-mutant enzymes in the class A B-lactamases- wider spectrum of activity
-confer resistance to monobactams (aztreonam), cerfotaxime, ceftazidime and other broad spectrum cephalosporins: not active against carbapenems (imipenem) or cephamycins (cefoxitin/cefotetan)
-primarily found in Klebsiella pneumoniae and oxytoca, E coli and P miribilis
-most are inhibited well by clavulanic acid and tazobactam
-resistance conferred to extended-spectrum penicillins, 3rd and 4th generation cephalosporins and aztreonam
-susceptible to carbapenases (imipenem) or cephamycins (cefoxitin/cefotetan)


Therapeutic Choices of ESBL's

-carbapenems (imipenem, ertapenem, meropenem) are the most effective therapeutic agents against ESBL-producing enterobacteriaceae


Amp C B-lactamases

-AmpC B lactamases differ from ESBLs
-they are cephalosporinases
-are resistant to available B- lectamase inhibitors
-hydrolyze the cephamycins (cefotetan, cefoxitin)

-AmpC is normally produced in low levels by many organisms and not associated with resistance but it can be produced at high levels to cause resistance

-high level production of AmpC usually causes resistance to all B-lactams except carbapenems and 4th generation cephalosporins

-chromosomal B-lectamases are part of the genotype of many organisms


Class C AmpC B-lactamases

-cephalosporinases, hydrolyze all beta lactam antibiotics except carbapenems and cefepime
-not inhibited by clavulanate and sulbactam
-inducible, most chromosomal in origin
-the ampC gene is present in 100% of isolates:
P-Providencia/P. aeruginose
C-Citrobacter freundii
H- Hafnia


Plasmid-Mediated AmpCs

-observed in Klebsiella, E. coli, salmonella, P. mirabilis, C. koseri
-usually constitutively expressed, but some enzymes are inducible and may be more dangerous clinically- CMY, FOX, and DHA families- crude mortality rate of 46%
-prevalence low by increasing- approx. 1/3 of US laboratories report finding isolates, 3.3-8.5 K pneumoniae in USA


Carbapenem-hydrolyzing B-lactamases

-the carbapenem-hydrolyzing enzymes are the most diverse group of all B-lactamases
-they have a breadth of spectrum unrivaled by other B-lactam hydrolyzing enzymes
-although referred to as carbapenemases many of the enzymes recognize almost all hydrolyzable B-lactams and are resistant to inhibition by B-lactam inhibitiors
-originally described only as chromosomally encoded, they are now plasmid encoded


Carbapenem-resistant enterobacteriaceae (CRE)

-the cabamenams include imipenem, meropenem, ertapenem and doripernem
-the carbapenase confers resistance to all B-lactams including penicillins, extended-spectrum cephalosporins, monobactrams, and carbapenems
-infections with CRE are difficult to treat and have been associated with mortality rates as high as 40-50%
-13% of Klebsiella species reported from central line-associated bloodstream infections and catheter associated UTIs


New Delhi Metallo-beta-lactamase

-the new superbug
-widespread in india, pakistan, and Bangladest
-ESBLs found in 70-90s of enterobacteriaceae in India
-now observed in the UK
-seen in E coli and K pneumoniae
-they are no antibiotics in the pipeline that have activity against NDM-1 producing Enterobacteriaceae


Resistance in Gram Posiives

-inducible clindamycin resistance

Group B Strep/ B-hemolytic Strep: erythromycin resistance and inducible clindamycin resistance

Enterococcus- VRE, amplicillin/penicillin resistance

Pneumonococci: multi-drug: penicillin, macrolide, and tetracycline resistance, fluroquinolone resistance


B- lactam resistance in staphylococci

-staphylococci resist attack by B lactam antibiotics in two ways
-production of B lactamases, enzymes that inactivate through hydrolysis of the B-lactam ring
-production of modified transpeptidase (penicillin binding proteins) targets that are impervious to antibiotic activity


Resistance to Penicillinase Resistant Penicillins

-may be the result of mutations in the PBP genes that lower binding affinity (penicillin-resistant pneumonococci)
-methicillin resistant S aureus (MRSA)
-organism has aquired the MecA gene which is responsible for the production of PBP 2a
-PBP 2a has structural changes that result in energetically unfavorable interactions between antibiotic and proteins, so that the active site serine is inactivated not at all or too slowly to effectively block cell wall synthesis and bacterial growth


Hospital vs Community Acquired MRSA

-age 68
-underlying disease 76%
-skin/soft tissue involvement: 37%
-respiratory tract involvement: 22%

-underlying disease 15%
-skin/soft tissue involvement: 75%
-respiratory tract involvement : 6%


Vancomycin Resistance

-while B lactam antibiotics inhibit cell wall synthesis by binding to the transpeptidase active site of PBP's
-Vancomycin binds to the C terminal D-Ala-D-Ala residue


Proposed Mechanism of VISA/hVISA

-resistance is not due to the acquistion of a resistance determinant via horizontal gene transfer but appears to be an adaptive multifactorial response to sublethal vancomycin exposure

-it has been proposed that the thicker, disorganized cell walls can actually trap vancomycin at the periphery of the cell, thereby blocking its action

-vancomycin can be recovered intact from the cell walls of VISA and hVISA isolates, indicating that the antibiotic is not being inactivated but merely sequestered by the bacteria


Aminoglycoside Resistance

-aminoglycosides are subject to modification and loss of antimicrobial activity in both gram-positive and gram-negative bacterial pathogens
-acquired resistance- decreased drug uptake, efflux pump activity, enzymatic modification of the drug
-intrinsic resistance- all enterococci have intrinsic resistance to aminoglycosides


Tetracycline Resistance

-the widespread use of tetracycline, both for human therapy and in animal feeds to promote growth, has been followed in recent years by an increased number of resistant strains
-thirty three different genes for tetracycline resistance have been identified
-the tetracycline resistance gene code for proteins that confer resistance by two main mechanisms: efflux pump, ribosomal protection (sometimes both)


Macrolide (erythromycin) resistance

-intrinsic: enterobacteriaceae exhibit decreased permability of the outer cell envelope to macrolides
-chromosomally encoded efflux pumps of several families can be responsible for macrolifes

mutational: mutations in genes for 50S ribosomal proteins

acquired: active efflux- msrA gene, decrease binding- erm genes


Sulfonamide Resistance

-Mutational: resistance or partial resistance by mutation, resulting in microbial overproduction of PABA or structural change in dihydropteroate synthase that produces an enzyme with lowered affinity for sulfonamide

-acquired: plasmid-mediated sulfonamide resistance has increased greatly in more recent years, often in conjunction with trimethoprim resistance


Resistance to Trimethoprim

-bacteria may develop trimethoprim resistance by several mechanisms, which can be chromosome- or plasmid-mediated
-trimethoprim resistance may be caused by:
changes in cell permeability
loss of bacterial drug-binding capacity
alteration in dihydrofolate reductase- clinically the most important- plasmid mediated


Fluoroquninolone Resistance

-fluoroquniolones include ciprofloxainm, norfloxacin, ofloxacin, and levofloxacin
-bacteria acquire resistance to quinolones by spontaneously occurring mutations in chromosomal genes that either: alter the target enzymes, DNA gyrase and topisomerase IV
-alter drug permeation across the bacterial cell membranes
-recently several plasmid-mediated quinolone resistance mechanisms have bween identified in clinical isolates of enterobacteriaceae


Summary: Genetics of Antibiotic Resistance

-bacteria may use a variety mechanisms or modes to escape the lethal action of action of antibiotics
-many bacteria may have several mechanisms of resistance available for a particular antibiotic- providing challenged in the laboratory for detection- some of these mechanisms may be reversible
-bacterial resistance to antibiotics is frequently regulated- and the modulation of gene expression enhances to survivability of bacteria