Alphabet Soup: MRSA – ESBL - CRE Jan Larmouth, MS, CIC Director, Infection Prevention and IV Resource SNHMC

Biochemistry is not scary. Points to remember … Biochemistry is not scary. We all live in a 3 dimensional world. Life happens at the molecular level.

Bacterial Cell structures Wall, plasmid, DNA

Classification Gram positive cocci Gram negative bacilli

Bacterial DNA Bacteria are comprised of only 1 cell. DNA within the cell = chromosomal and plasmid. DNA is constantly being clipped and shuffled between bacteria, including from different species. The DNA sequence of bases determines what protein is going to be manufactured.

A day in the life … These are the steps to produce proteins within a bacterial cell: DNA splits open and one section of the ladder is matched up with other base pairs in the cytoplasmic soup to form a copy. This copy then passes through a ribosome that “translates” the genetic code into a protein.

Enzymes – “ase” Make things happen! One kind of protein is an enzyme. Amylase in our mouths begins to break down starches. Each enzyme will only work on a particular chemical called a substrate. The reason is that it has to fit into an active site like a lock and key in order to work. Left = highly simplified picture. Right – a 2D model of a 3 D enzyme.

Antibiotics = physical structure Different classes have different chemical structures Penicillin (methicillin) Just as enzymes have a 3D structure, so do antibiotics. Some are very simple and others are very complex.

Antibiotics Beta-lactams (cephalosporins) Some of the most commonly used antibiotics are the cephalosporins, and there are many different chemical structures of these compounds.

How ABX work and how bugs fight back! Antibiotics disrupt an essential process that is necessary for cell life: Cell wall synthesis DNA replication Organisms change their structure and produce chemicals that protect them from the ABX The goal of a bacteria is to stay alive and reproduce. The goal of an antibiotic is to disrupt the processes needed to sustain life and either kill or inhibit reproduction of the bacteria. This can be accomplished in several different ways.

These are the processes that keep a bacteria alive and these are the classes of antibiotics that can disrupt those processes.

Staphylococcus aureus Gram positive cocci Clonal organism – not many spontaneous mutations Normal skin flora – 30% of population = MSSA Methicillin worked well until around 1960s Let’s start with a simple example: How did Staph aureus become resistant to methicillin?

Penicillins – mechanism of action TP – transpeptidase -lactam ring binds to TP (penicillin binding protein) No cross links are formed = cell ruptures The cell maintains its cell wall by constantly tearing it down and building it back up. One of the enzymes necessary for this process is a transpeptidase. This enzyme helps form the cross links of molecules that strengthen the cell wall. If the bacteria tries to do this in the presence of penicillin, the beta lactam ring binds to the transpeptidase so the crosslinks don’t form and the cell is weakened and it ruptures.

Change the structure and you’re safe! MRSA are genetically slightly different - acquired the mec A gene = PBP = different Methicillin doesn’t fit anymore But eventually, S. aureus changed it’s wall structure so that penicillin couldn’t get into the cell but Methicillin could. Eventually, the structure of the penicillin binding proteins changed again, and methicillin was not effective to disrupt the cell wall.

Extended Spectrum Beta-Lactamase (Enterobacteriaceae) Nomenclature differs: not the bug but the enzyme Not one genus/species Many species: E. coli, Klebsiella pneumoniae, Enterobacter cloacae, Proteus mirabilis, etc. Most common: E. coli; K. pneumoniae Many more gram negatives isolated from clinical infections than gram positives and there are many more pathogenic species of GNR.

Evolution of beta lactam ABX First Generation: cefazolin, cefalexin, cefadroxil Second Generation: cefamandole, cefoxitin, cefaclor, cefuroxime, loracarbef, cefotetan Third generation: cefotaxime, cefpodoxime, ceftizoxime, ceftriaxone, ceftazidime, cefoperazone Fourth generation: cefepime, cefozopran, cefpirome, cefquinome Generations have to do with marketing rather than anything else but the newer drugs are more complex in chemical structure.

Why so many? Activity is different Generation1: Staph and Enterobacteriaceae Generations 2, 3 and 4: Structure able to resist beta-lactamases – broader spectrum

Structure of ABX

Chromosomal DNA codes for L An important fact to remember is that GNR are promiscuous. They love to share their DNA with their neighbors. This slide shos hoe DNA can be passed from 1 cell to another, even between different species.

Passing on the resistance Extended spectrum = the enzyme produced inactivates a 3rd or 4th generation cephalosporin The bugs just keep getting smarter! Can be passed to other bacteria on a plasmid – a highly mobile circle of DNA

CREs – a step above ESBLs Carbapenem resistant Enterobacteriaceae Same organisms as ESBLs E. coli, Klebsiella, etc. But resistant to a whole new class of ABX If we can’t use cephalosporins, then we have to use a different class of antibiotics.

The Carbapenems Imipenem – 1980 – Gram Pos and Gram Neg - 3X a day (IV/IM) Meropenem – Mostly Gram Neg - 3X a day Ertapenem – slightly narrower spectrum – 1X Doripenem – Complex abdominal infections Carbapenems are used as the last resort antibiotics which means that they are used only when alternative options have failed to achieve any noticeable results.

They are still β-lactam ABX Although they still contain the beta lactam ring, they offer properties of inhibiting the production of beta lactamase enzymes within the cell. But again, the bacterial cells have developed mechanisms to protect themselves.

Mechanism of Action/Resistance

What does all this mean? Stop the spread. Basic IP principle reinforcement. Prevent the use of unnecessary ABX . Consider narrow rather than broad spectrum.