Antibiotics and Antibiotics Resistance
Disinfectants: antimicrobial agents that are applied to inanimate object (e.g. floors)
Antiseptics: antimicrobial agents that are sufficiently nontoxic to be applied to
living tissues (e.g. hand sanitizers)
Antibiotics: antimicrobial agents produced by bacteria and fungi that are exploited
by humans (delivered topically and internally)
Antibiotics come from nature and we can modify them to improve stability and
activity. Microorganisms use these antibiotics to compete with each other in a
specific niche, sometimes developing resistance.
Alexander Fleming (1881-1955)
He discovered penicillin in 1928, which was produced from Penicillin notatum. He
found that colonies of Staphlococci were not able to grow around a contaminating
mold. He was rewarded the Nobel Prize as a result of this discovery.
Antibiotics represent our most effective therapeutic against bacterial infection. The
availability of antibiotics enables cancer chemotherapy, organ transplantation, and
all invasive surgeries. Antibiotic mortality rate is much lower than that of the pre-
antibiotic mortality rate with resistance, we might be moving to the post-
antibiotic resistant era.
There are two major problems:
1. Diminished interest from pharmaceutical companies to develop new
antibiotics companies are driven to make money, and it is a huge venture
to make antibiotics (15-20 years; lots of money). Every time a new antibiotic
is introduced in the clinic, the body usually develops resistance right away
(most drugs fail, so its just unfeasible).
2. Bacterial resistance to antibiotics always happens
Antimicrobial discovery has significantly slowed down because immediate
resistance is a huge problem. The 1940-50’s was the golden era for antibiotics.
Antibiotic resistance is ancient
Antibiotics and antibiotic resistance has been around for a very long time in nature,
far before the use of antibiotics in a clinical setting now we are selecting for a
resistant organism. Ancient DNA from frozen permafrost (~30,000 years old) from the Yukon has found
antibiotic resistance genes. People used a technique to sequence all the genetic
material that you can find in the soil (doesn’t involve culturing). They were able to
clone a bank of resistant genes to confirm resistance, so now we are selecting from
Note: Through air traffic, resistant-selected phenotypes can go global.
How do antibiotics work?
Antibiotics either kill bacteria, or stop them from growing (not all antibiotics kill
bacteria). For bacteriostatic antibiotics, the immune system must take over to kill
Measuring Antibiotic Activity
The minimum inhibitory concentration (MIC) measures how active the antibiotic is
for a particular organism. MIC = lowest concentration of agent that inhibits growth.
There are two methods:
1. Old-school method involves a series of culture tubes with varying
concentration of agent, where you increase the concentration of antibiotic to
see what concentration kills the culture the lower the MIC, the more
potent the antibiotic is
2. Newer method involves antibiotic strips placed on a plate spreaded with
bacteria. The strips have different antibiotics (more concentrated on the
ends and less in the center zone of inhibition). This is a faster method.
How do antibiotics work?
Antibiotics target essential bacterial components, which are unique to bacteria and
separate from humans:
Cell wall synthesis
Protein synthesis DNA/RNA synthesis
Cell membrane alteration
Contains a “B-lactam ring”
Function to inhibit cell wall synthesis in bacteria
B-lactams bind the bacterial “penicillin-binding proteins” (PBP)
PBPs are transpeptidases
No peptide cross-links = weak cell wall = cell death
But some bacteria can produce B-lactamase (an enzyme that destroys the
ring and thus the antibiotic) B-lactamase producing bacteria are resistant
Contains a “B-lactam ring”
Chemically modified penicillin (cant be cleaved by B-lactamases)
But some bacteria can produce a different PBP (e.g. PBP2a, genetically
encoded by mec)
PBP2a doesn’t bind methicillin (or other B-lactams) bacteria are resistant
Bacterial Cell Wall Synthesis
Transpeptidase is responsible for cell wall synthesis, and B-lactams are antibiotics
that inhibit it such that bacteria will not be able to produce a cell wall and die.
Bacteria contain B-lactamase, which inhibits B-lactams, thus disallowing it from
inhibiting transpeptidase (PBP). Methicillin also inhibits transpeptidase and is
resistant to B-lactamase. Bacteria are able to overcome this again by producing a
different PBP (PBP2a) such that penicillin and methicillin can’t bind. Vancomycin
Vancomycin inhibits cell wall synthesis in Gram positives often a drug of “last
resort” (e.g. HA-MRSA). It is a glycopeptide antibiotic that targets G bacteria (cant
get through the outer membrane in G ). Vancomycin binds the peptide linkage at
terminal D-Ala-D-Ala residues and inhibits transpeptidation. Resistance genes
change these to D-Ala-D-Lac and vancomycin can no longer bind resistance is
encoded by the van genes.
Protein Synthesis Inhibitors
Eukaryotes contain 80S (40S+60S) ribosomes, and bacteria contain 70S (30S+50S)
ribosomes. Many antibiotics target bacterial ribosomes and block translation.
30S inhibitors (e.g. Tetracycline, Kanamycin)
50S inhibitors (e.g. Erythromycin, Chloramphenicol)
Folic acid synthesis inhibitors
Folic acid is a vitamin (B9) for humans. Bacteria need folic acid for thymidine
synthesis, and they can’t absorb folic acid so they must synthesize their own.
Inhibition of folic acid synthesis blocks DNA replication. DNA/RNA synthesis inhibitors
Fluoroquinolones: interfere with DNA gyrase needed for supercoiling of DNA
Rifampicin: inhibits bacterial RNA polymerase
Cell Membrane Alteration