LIFE3035: PATHOGENS
Question 1: Comparing different bacterial pathogens- explain the different mechanisms by
which antibiotic resistance is acquired and selected for (70% of mark). Outline what general
strategies you would use to combat antibiotic resistance (30% of mark).
Since the first discovery in 1928, antibiotics quickly became the “wonder drug” and
revolutionised modern medicine. However, misuse and overuse of antibiotics have led to the
evolution of pathogens to resist the drugs used to combat them. Drug resistance is a growing
global threat to human health, which causes at least 700,000 deaths each year, and negatively
impacts the economy. Not only can bacteria develop a mechanism of resistance for themselves,
which is usually encoded by genes, but they can also transfer these genes to other bacterial
species. This essay will explain the different mechanisms by which antibiotic resistance is acquired
and selected for in different bacterial pathogens. Based on that, strategies to combat antibiotic
resistance will be proposed and discussed.
Since the first report of penicillin resistance, studies have continuously identified several genes
and vectors of transmission which pathogens possess intrinsically or extrinsically (acquired from
other species). These genes’ products could protect the target sites antibiotics aim for (examples
in figure 1) or cause changes to antibiotics themselves. In general, they represent three
mechanisms of resistance: prevention of access to target, physical changes in antibiotic targets by
mutations, and direct modification of antibiotics. Different pathogens employ different methods
of defence; therefore, to avoid multiple changes, resistance is most common between antibiotics
of the same class. However, nowadays, bacteria can accumulate several resistance genes or
increase the expression of multidrug efflux pumps and become resistant to a wide range of
antibiotics [1]. ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae,
Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) pathogens are
well-known examples with growing multi-drug resistant properties. The mechanisms of antibiotic
resistance in some of the species among the ESKAPE group, along with Mycobacterium
tuberculosis, will be explained and compared in this essay.
Figure 1
, Mycobacterium tuberculosis (Mtb) is a slow-growing gram-positive bacterium which is the
main cause of tuberculosis. Mtb has both intrinsic and acquired resistance mechanisms to
tuberculosis drugs such as isoniazid, rifampin and streptomycin. Mtb’s mycolic acid-containing cell
wall serves as the permeability barrier for many antibiotics due to its hydrophobicity [2].
Previously, the production of β-lactamase encoded by blaC and blaS was considered the main
reason for Mtb’s resistance to β-lactam antibiotics. However, in a more recent study, researchers
investigated genes responsible for β-lactam antibiotics resistance of Mtb. They found a mutant in
front of the gene bcg0231 and rv0194 encodes a new multidrug efflux pump of Mtb [3]. On the
other hand, Mtb has acquired resistance mechanisms by mutations at chromosomal loci, which
modify the sequence of the antibiotic targets and could lead to multi-drug resistance. For
example, mutations at gyrA and gyrB, genes encoding for DNA gyrase, are associated with
fluoroquinolones resistance[4]. Since DNA gyrase is responsible for DNA coiling, fluoroquinolones
target DNA gyrase to inhibit the growth of tubercle bacilli, making them the important second-line
tuberculosis drugs.
Mutations at different genes such as katG, rpoB, pncA lead to the resistance to drugs
including Isoniazid, Rifampin, Pyrazinamide, respectively [5-9]. The third type of resistance is
caused by salicylates, a group of chemicals derived from salicylic acid and are naturally found in
some foods. Salicylates were thought to activate multiple antibiotic resistance (Mar) genes in E.
coli but the mechanism in Mtb remained unclear[10]. Two MarA homologs were found in the M.
smegmatis genome so it is proposed that Mtb possess a Mar-like regulatory mechanism [11].
Similarly, still remaining unknown is the mechanism of persistent and dormant Mtb, which survive
extensive chemotherapy[12]. In summary, the emergence of multidrug resistant Mtb is still going
but Mtb genome as well as drug screening techniques are now available for further studies.
Different from Mtb, enteric bacteria like E. coli and Salmonella spp are more likely to acquire
new mutations by gene transfer rather than by a de novo mutation in that bacterium. It is likely
due to the genetic systems that prevent mutation (DNA repair and protection proteins). Horizontal
gene transfer (HGT) is the method an organism uses to take up genes on plasmids of other
organisms of different species. A typical example for HGT is, sadly, the reason for the inactivation
of most β-lactam antibiotics. Kluyvera species found in soil have passed on the CTX-M genes on its
chromosome to its neighbouring bacteria[13]. Later on, numerous variants of CTX-M genes were
found to produce extended-spectrum β-lactamases (ESBLs), the enzymes that degrade not only β-
lactam antibiotics but also cephalosporins and monobactams[14, 15]. A study in 2012 estimated
that at least 109 members of CTX-M family were found in 26 bacterial species, most prevalent in E.
coli and K. pneumoniae [16]. The rapid spread of CTX-M-type ESBL-producing bacteria is one of the
concerns in the epidemiology of antibiotic resistance.
In 2011, Shiga toxin-producing E. coli O104:H4 (STEC O104:H4) was responsible for an outbreak
of gastroenteritis in Germany. STEC O104:H4 is a classic enteroaggregative E. coli (EAEC)
containing a new virulence – Shiga toxin – via a bacteriophage encoding stx2 [17]. STEC was also
found to possess the CTX-M type ESBLs, which made the outbreak even worse because of the
resistance to multiple antibiotics. However, STEC had been previously known to omit a region of
non-pathogenic genes between cadA and melA[18]. This agrees with a finding in 2019 which
showed thousands of gene acquisition and loss over the evolution of E. coli [19].
Despite the possibility to acquire antibiotic resistant genes, E. coli and other Enterobactericae
could still intrinsically resist the activity of antibiotics via limiting antibiotics entry through porins.
Moreover, the down regulation of porin genes were shown to significantly resist drugs like
carbapenems and cephalosporins. In contrast, the over-expression of efflux pump, especially MDR
pumps, increases the levels of resistance. It is because regulators of the multidrug pump genes are
encoded alongside a repressor of the Mar proteins [20]. Therefore, mutations in the repressor of
Mar proteins lead to the transcription of Mar and the overexpression of multidrug efflux pumps.
Question 1: Comparing different bacterial pathogens- explain the different mechanisms by
which antibiotic resistance is acquired and selected for (70% of mark). Outline what general
strategies you would use to combat antibiotic resistance (30% of mark).
Since the first discovery in 1928, antibiotics quickly became the “wonder drug” and
revolutionised modern medicine. However, misuse and overuse of antibiotics have led to the
evolution of pathogens to resist the drugs used to combat them. Drug resistance is a growing
global threat to human health, which causes at least 700,000 deaths each year, and negatively
impacts the economy. Not only can bacteria develop a mechanism of resistance for themselves,
which is usually encoded by genes, but they can also transfer these genes to other bacterial
species. This essay will explain the different mechanisms by which antibiotic resistance is acquired
and selected for in different bacterial pathogens. Based on that, strategies to combat antibiotic
resistance will be proposed and discussed.
Since the first report of penicillin resistance, studies have continuously identified several genes
and vectors of transmission which pathogens possess intrinsically or extrinsically (acquired from
other species). These genes’ products could protect the target sites antibiotics aim for (examples
in figure 1) or cause changes to antibiotics themselves. In general, they represent three
mechanisms of resistance: prevention of access to target, physical changes in antibiotic targets by
mutations, and direct modification of antibiotics. Different pathogens employ different methods
of defence; therefore, to avoid multiple changes, resistance is most common between antibiotics
of the same class. However, nowadays, bacteria can accumulate several resistance genes or
increase the expression of multidrug efflux pumps and become resistant to a wide range of
antibiotics [1]. ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae,
Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) pathogens are
well-known examples with growing multi-drug resistant properties. The mechanisms of antibiotic
resistance in some of the species among the ESKAPE group, along with Mycobacterium
tuberculosis, will be explained and compared in this essay.
Figure 1
, Mycobacterium tuberculosis (Mtb) is a slow-growing gram-positive bacterium which is the
main cause of tuberculosis. Mtb has both intrinsic and acquired resistance mechanisms to
tuberculosis drugs such as isoniazid, rifampin and streptomycin. Mtb’s mycolic acid-containing cell
wall serves as the permeability barrier for many antibiotics due to its hydrophobicity [2].
Previously, the production of β-lactamase encoded by blaC and blaS was considered the main
reason for Mtb’s resistance to β-lactam antibiotics. However, in a more recent study, researchers
investigated genes responsible for β-lactam antibiotics resistance of Mtb. They found a mutant in
front of the gene bcg0231 and rv0194 encodes a new multidrug efflux pump of Mtb [3]. On the
other hand, Mtb has acquired resistance mechanisms by mutations at chromosomal loci, which
modify the sequence of the antibiotic targets and could lead to multi-drug resistance. For
example, mutations at gyrA and gyrB, genes encoding for DNA gyrase, are associated with
fluoroquinolones resistance[4]. Since DNA gyrase is responsible for DNA coiling, fluoroquinolones
target DNA gyrase to inhibit the growth of tubercle bacilli, making them the important second-line
tuberculosis drugs.
Mutations at different genes such as katG, rpoB, pncA lead to the resistance to drugs
including Isoniazid, Rifampin, Pyrazinamide, respectively [5-9]. The third type of resistance is
caused by salicylates, a group of chemicals derived from salicylic acid and are naturally found in
some foods. Salicylates were thought to activate multiple antibiotic resistance (Mar) genes in E.
coli but the mechanism in Mtb remained unclear[10]. Two MarA homologs were found in the M.
smegmatis genome so it is proposed that Mtb possess a Mar-like regulatory mechanism [11].
Similarly, still remaining unknown is the mechanism of persistent and dormant Mtb, which survive
extensive chemotherapy[12]. In summary, the emergence of multidrug resistant Mtb is still going
but Mtb genome as well as drug screening techniques are now available for further studies.
Different from Mtb, enteric bacteria like E. coli and Salmonella spp are more likely to acquire
new mutations by gene transfer rather than by a de novo mutation in that bacterium. It is likely
due to the genetic systems that prevent mutation (DNA repair and protection proteins). Horizontal
gene transfer (HGT) is the method an organism uses to take up genes on plasmids of other
organisms of different species. A typical example for HGT is, sadly, the reason for the inactivation
of most β-lactam antibiotics. Kluyvera species found in soil have passed on the CTX-M genes on its
chromosome to its neighbouring bacteria[13]. Later on, numerous variants of CTX-M genes were
found to produce extended-spectrum β-lactamases (ESBLs), the enzymes that degrade not only β-
lactam antibiotics but also cephalosporins and monobactams[14, 15]. A study in 2012 estimated
that at least 109 members of CTX-M family were found in 26 bacterial species, most prevalent in E.
coli and K. pneumoniae [16]. The rapid spread of CTX-M-type ESBL-producing bacteria is one of the
concerns in the epidemiology of antibiotic resistance.
In 2011, Shiga toxin-producing E. coli O104:H4 (STEC O104:H4) was responsible for an outbreak
of gastroenteritis in Germany. STEC O104:H4 is a classic enteroaggregative E. coli (EAEC)
containing a new virulence – Shiga toxin – via a bacteriophage encoding stx2 [17]. STEC was also
found to possess the CTX-M type ESBLs, which made the outbreak even worse because of the
resistance to multiple antibiotics. However, STEC had been previously known to omit a region of
non-pathogenic genes between cadA and melA[18]. This agrees with a finding in 2019 which
showed thousands of gene acquisition and loss over the evolution of E. coli [19].
Despite the possibility to acquire antibiotic resistant genes, E. coli and other Enterobactericae
could still intrinsically resist the activity of antibiotics via limiting antibiotics entry through porins.
Moreover, the down regulation of porin genes were shown to significantly resist drugs like
carbapenems and cephalosporins. In contrast, the over-expression of efflux pump, especially MDR
pumps, increases the levels of resistance. It is because regulators of the multidrug pump genes are
encoded alongside a repressor of the Mar proteins [20]. Therefore, mutations in the repressor of
Mar proteins lead to the transcription of Mar and the overexpression of multidrug efflux pumps.
