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

2. 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.

3. Bacterial Cell structures
Wall, plasmid, DNA

4. Classification
Gram positive cocci Gram negative bacilli

5. 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.

6. 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.

7. 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.

8. 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.

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

10. 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.

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

12. 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?

13. 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.

14. 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.

15. 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.

16. 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.

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

18. Structure of ABX

19. 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.

20. 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

21. 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.

22. 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.

23. 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.

24. Mechanism of Action/Resistance

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