Host immunity in multidrug-resistant

bacterial infections

Multidrug-resistant Bacteria (MDR)

输入标题

Drug-resistant: microorganisms with diminished susceptibility to, or loss of, key drugs used in clinical therapy (routine).

Multidrug-resistant organisms (MDRO): mainly bacteria that are resistant to three or more classes of antimicrobial

drugs used in clinical practice at the same time, at least one of each class: structurally different and with different

mechanisms of action).

MDR

Multidrug resistance includes extensive drug resistance (XDR) and pan-drug resistance (PDR).

XDR: insensitive to all antimicrobials except those in groups 1-2 (mucins or cyclins).

PDR: insensitive to all drugs in all antimicrobial classes.

XDR

Not MDR

PDR

Bacterial resistance is the result of evolutionary selection of bacteria, and bacterial resistance is a natural phenomenon

that has been exacerbated by the misuse of antibiotics.

输入标题

Patients (infectious diseases)

Pathogenicity sent for testing

Laboratory Microbiology

bacterial culture

bacteria-positive

bacteria-negative

multidrug-

resistant

non-multidrug-resistant

test report issued and labelled

by the laboratory department

general patient consultation

Guidance, supervision and

inspection by the Hospital

Infection Management Section

clinical department

contact quarantine

clinical symptoms improved or cured

implementation of contact quarantine

输入标题

1

. Carbapenem - resistant Enterobacteriaceae(CRE)

(i) Escherichia coli; (ii) Klebsiella pneumoniae

2

. Methicillin-resistant Staphylococcus Aureus(MRSA)

3

. Vancomycin-resistant Enterococcus(VRE)

(i) Enterococcus faecium; (ii) Enterococcus faecalis

4

. Cabapemne-resistant Acinetobacter Baumannii(CRAB)

5. Carbapenem-resistant antimicrobials Pseudomonas aeruginosa(CRPAE)

MRSA

VRE

CRE

CR-KP

输入标题

Factors

Specific Reasons

Elderly

Elderly people are susceptible to hospital-acquired multidrug-resistant bacteria due to aging of

organs and tissues, decline in physiological and metabolic functions, and low immune function.

Immunocompromised Including patients with diabetes mellitus, chronic obstructive pulmonary disease, cirrhosis of the liver,

uremia, patients with tumours treated with immunosuppressants for a long period of time, receiving

radiotherapy or chemotherapy, etc.

Invasive operations

Invasive operations easily destroy the barrier of the skin and mucous membranes and damage

the host's defence system, which can bring pathogenic bacteria into the organism leading to

exogenous infections at the same time can also cause endogenous infections caused by bacterial

translocation in the body.

Antibiotic treatments Recent treatment with 3 or more antimicrobial drugs (within 90 days).

Previous hospitalisation Longer hospital stays increase the risk of hospital-acquired infections and increase the chance of

multidrug-resistant infections.

Past medical history

Previous history of MDR colonisation or infection, etc.

输入标题

Higher morbidity and

mortality than in patients

infected or uninfected

with sensitive organisms

Contact transmission:

Most common mode of

transmission

0

1

MDR is consistent with the

clinical presentation of

infections with sensitive

strains of bacteria, but the

choice of antimicrobials is

extremely limited.

0

2

Droplet transmission:

Includes coughing,

suctioning, and other

a e r o s o l - f o r m i n g

operations

Prolonged hospital stays,

i nc r e a se d cost s f or

diagnosis and treatment of

infections

0

3

Increased risk of adverse

reactions to antimicrobials

0

4

Airborne:

Become a source of

communication

when air conditioning

vents are contaminated

by MDROs

0

5

Mechanisms of action of antibacterial agents

&

Mechanisms of multidrug-resistant bacteria drug resistance

&

Mechanisms of immune response of bacteria infection

Mechanisms of action of antibacterial agents

Most antimicrobial agents used for the

treatment of bacterial infections may be

categorized according to their principal

mechanism of action.

There are 4 major modes of action:

(

1) interference with cell wall synthesis

2) inhibition of protein synthesis

3) interference with nucleic acid synthesis

4) inhibition of a metabolic pathway

Neu, H. C. (1992). The crisis in antibiotic resistance. Science, 257(5073), 1064-1073.

General antimicrobial resistance mechanisms

General antimicrobial resistance mechanisms. Antimicrobial resistance

mechanisms fall into four main categories:

(

1) limiting uptake of a drug;

2) modifying a drug target;

3) inactivating a drug;

4) active drug efflux.

Gram negative bacteria make use of all four main mechanisms.

Gram positive bacteria less commonly use limiting the uptake of a drug.

General antimicrobial resistance mechanisms.

β-Lactam resistance in Gram-positive bacteria

β-Lactamantibiotics exert bactericidal activity by

covalently binding to and inactivating enzymes

known as penicillin-binding proteins (PBPs),

resulting in interference with the synthesis and

remodeling of the bacterial peptidoglycan.

In Gram-positive bacteria, β-lactam resistance

primarily occurs as a result of alteration of PBPs,

with enzymatic degradation as a minor pathway.

β-Lactam resistance in Gram-positive bacteria

β-lactam resistance in Gram-negative bacteria

β-Lactam resistance in Gram-negative bacteria can arise

through three possible mechanisms:

(

1) Alteration of PBPs

(

2) Production of βlactamases

(3) Limited access to target PBPs

Any modulation that restricts entry (porin loss) or result in

extrusion (efflux pumps) of β-lactamantibiotics will confer

antibiotic resistance.

Production of β-lactamases is the most prevalent resistance

mechanism.

β-lactam resistance in Gram-negative bacteria

Mechanisms of immune response---innate response

Pathogen-associated

molecular patterns

recognization

Innate response of bacterial infection

The innate immune system is the early line of defense against bacterial pathogens.

Cells participating in the innate immune defense are blood monocytes, tissue macrophages, dendritic cells (DCs),

NK cells and NK T cells. Bacteria are recognized by the cells when well-conserved bacterial structures known as

pathogen-associated molecular patterns (PAMPs) are recognized by specific receptors.

Mechanisms of immune response---innate response

Secreting of cytokines

Innate response of bacterial infection

Through the activation of signal pathway, transcription factors will be activated, enter the nucleus and combine with

DNA, thus regulating the expression of specific genes. These genes encode cytokines.

Cytokine secretion: Activated immune cells begin to synthesize and secrete cytokines and release them into the

extracellular environment. These cytokines are different, they can be inflammatory mediators, chemokines,

lymphokines and so on.

Mechanisms of immune response---innate response

Functions of cytokines

Innate response of bacterial infection

The main goal of secreted cytokines is to orchestrate immune responses so that bacterial load is contained. Cytokines

are proteins that act in either a proinflammatorily or anti-inflammatory way.

Mechanisms of immune response---adaptive response

The adaptive immune

response to a bacterial

stimulus requires the

presentation of the bacterial

antigen from DCs and

monocytes that behave as

antigen-presenting cells to

naive T (Th0) cells.

Adaptive response of bacterial response

Th1: secrete TNF-a, IL-2 and IFN-g and prime pro inflammatory

responses to promote phagocytosis of bacterial pathogens;

Th2: secrete IL-4, IL-6 and IL-10 and prime anti-inflammatory responses;

T17: secrete IL-17 to attract neutrophils and prime efficient phagocytosis;

Tregs: prime anti-inflammatory responses.

Multidrug-resistant bacteria and bacteria

Immune escape mechanism of multidrug-resistant bacteria:

1. Virulence factors: Multidrug-resistant bacteria usually produce some virulence factors, such as exotoxin,

endotoxin and protease, which can destroy host cells, interfere with immune response and inhibit phagocytosis.

They can inhibit the inflammatory response of the host immune system and reduce the ability to recognize and

destroy bacteria.

2. Antigenic variation: Multidrug-resistant bacteria may escape the recognition of host immune system by

changing their surface antigen structure or expressing different antigens. This antigen variation makes it difficult

for the host immune system to recognize and produce specific immune response, thus making it more difficult for

bacteria to be eliminated.

3. Biofilm formation: Multidrug-resistant bacteria sometimes form biofilm, which is a membrane structure

composed of multiple layers of polymers, which can protect bacteria from the attack of host immune system.

Biofilm provides a physical barrier to prevent contact and phagocytosis of immune cells, thus increasing the

survival and replication of bacteria.

......

STRATEGIES EMPLOYED BY MDR BACTERIA TO EVADE

HOST IMMUNE RESPONSE

STRATEGIES EMPLOYED BY MDR BACTERIA TO EVADE

HOST IMMUNE RESPONSE

•

Inhibition of maturation of phagosome

Escape from oxidative killing by neutrophils

Neutralization of AMPs

Blockade of antigen presentation

Immunosuppression

Escape from autophagy and apoptosis

Circumvention of complements and cytokine cascade

Certain strains of multidrug-resistant bacteria have the ability to evade the immune system, creating challenges for

immune cells to accurately identify and eliminate them.

Inhibition of Maturation of Phagolysosome

The invaded bacteria are captured by the macrophage through a process known as

phagocytosis and in macrophage the phagosome fuses with lysosome to degrade

the bacteria.

Phagolysosome is a structure formed by the fusion of phagosome and lysosome.

Once MDR Bacteria invade the host cells, it replicate inside the host cells by

arresting the maturation of the phagosome. Enabling the bacterium to avoid

exposure to hydrolases, low pH conditions of lysosomes, and various other

lysosomal components that can kill the bacteria.

•

Inhibit phagosome acidification: PtpA

•

Inhibit phago-lysosome formation: RAB22a

Inhibit interferon production: CGAS

Escape from Oxidative Killing by Neutrophils

Mycobacterium tuberculosis can exhibit multidrug resistance.

Neutrophils can eliminate pathogenic bacteria in an oxidative manner at the site of

infection.

MDR Bacteria survival was also accompanied by the necrotic cell death of infected

neutrophils which depends on the production of radical oxygen species (ROS).

Mycobacterium tuberculosis

RD1 (Region of Difference 1) was related to the immune escape ability.

The RD1 region includes several genes, of which EsxA and EsxB are two important

genes involved in cell lysis and immune escape, and the complex they form is called the

"ESX-1 secretion system."

In the context of neutrophil lysis, Mycobacterium tuberculosis releases EsxA and EsxB

proteins through the ESX-1 system, and EsxA and EsxB proteins form pores that rupture

the membrane structure.

Due to the presence of RD-1, there is disruption of the membrane, leading to the leakage

of the radical products (ROS, NADPH-, Oxidase, MPO) into the cytosol. – Necrosis

Neutralization of the Antimicrobial Peptides (AMPs)

The AMPs are effector molecules of innate immunity, which is the body's first line of

defense against invading microorganisms.

Antimicrobial activity against Gram-positive, Gram-negative bacteria, and fungi is

the major mode of action for AMPs.

The initial contact between the AMPs and the target organism is electrostatic, as most

bacterial surfaces are anionic.

Some MDR Bacteria can circumvent the innate immune response by inhibiting the

function of AMPs.

MDR Bacteria with enhanced lysX expression demonstrated improved intracellular

survival.

In particular, the alteration of negative charges on the cell surface is carried out by the

lysX gene, which catalyses the transfer of lysine to phosphatidylglycerol, resulting in

an overall positive charge on the bacterial cytoplasmic membrane.

Antimicrobial Peptides (AMPs)

Current Pharmaceutical Biotechnology, 2022, 23, 1704-1720

Interference with Antigen Processing and Presentation

Some MDR Bacteria (for example Myobacteria) uses certain strategies to

manipulate the host's immune responses to persist within the host. The exact

mechanism that interferes with the antigen presentation is poorly understood,

although, as reported, mycobacterial infection of antigen-presenting cells

(APCs) interferes with MHC II antigen presentation in vitro conditions.

Inhibition of Apoptosis and Autophagy

Host cell apoptosis and the virulence of bacteria are interrelated.

Various components that play their role in virulence, For example, miR-30 regulates the apoptotic response of

macrophages in Mtb.

The overexpression of miR-30 suppresses bacteria elimination and is achieved by inhibiting autophagy.

Escape from the phagosome before the fusion begins

Some MDR Bacteria can simply escape from the phagosome before fusion occurs.

By using this strategy and then undergo unrestricted growth in the cytoplasm of the host cell.

Approaches for the treatment of infections due to

multidrug-resistant bacterial pathogens

only 18 molecules fulfill at

least one of innovation

criteria for antimicrobial

drugs (new target, absence

of known cross-resistance,

unique mode of action or

new class)

Escape from the phagosome

before the fusion begins

Beyer P, Paulin S. The antibacterial research and development pipeline needs

urgent solutions. ACS Infect. Dis. 6(6), 1289–1291 (2020).

Clinical Antibacterial Pipeline

➢

Host defence peptides (HDPs) are an evolutionarily ancient

component of the innate immune system of most multicellular

organisms.

In recent years potent immuno-modulatory properties of HDPs

have been characterised and suggested to be an important part

of their biological function.

A wide range of HDPs from different species have been shown

to act as chemoattractants for cells of innate and adaptive

immunity.

Human cathelicidin LL-37 attracts neutrophils,monocytes,

T cells, and mast cells,using formyl peptide receptor-like 1

(FPRL1), and a distinct Gi-coupled receptor.Optimal

chemotactic activity of LL-37 is observed in a

concentration range that can be reached in vivo under

inflammatory conditions.

A Nijnik & REW Hancock (2009) Host defence peptides: antimicrobial and immunomodulatory activity and potential applications for tackling antibiotic-resistant infections, Emerging Health

Threats Journal, 2:1, 7078.

➢

Besides having direct antimicrobial effects, defensins posses a variety of additional functions related to host defense.

In addition to direct activation of innate host defense mechanisms, some PRRs are coupled to the induction of adaptive

immune responses.

➢

Stimulation of the innate immune system by HDPs may offer a conceptually novel and advantageous approach to defeat

invading pathogenic organisms. As innate immunity is rapidly activated and involves a multitude of nonspecific effector

cells and mediators, immunomodulation may be used to target a broad spectrum of pathogens while at the same time

reducing the risk of resistance development.

Thomas Kruse & Hans-Henrik Kristensen (2008) Using antimicrobial host defense peptides as anti-infective and immunomodulatory agents, Expert Review of Anti-infective Therapy, 6:6, 887-895

➢

Bacteriophages are anobligate parasites that

naturally infects bacteria and destroy their bacterial

host upon infecting them. Phages are very specific

to their host and have recently emerged as a novel

therapeutic choice as it can successfully kill several

pathogens either alone or in combination with

antibiotic therapy.

➢

One of the major applications of bacteriophages has

been their utilization for the treatment of MDR and

XDR T u-berculosis (MDR-TB and XDR-TB) for

which there are extremely limited therapeutic

options available.

Several research groups have investigated

bacteriophages as an alternative to antibiotics

adaptive immunity.

➢

One of the foremost examples is adjuvant

immunotherapy against drug-resistant TB.

Adjuvant immunotherapy includes host-directed

immunomodulation with immuno-adjuvants, use of

recombinant and

cytokines,

corticosteroids

vaccination as promising antimicrobial therapies.

Host-directed therapies aimed at modulating immune responses in the tuberculous lung.

Alimuddin Zumla, Martin Rao (2016) Potential of immunomodulatory agents as adjunct host-directed therapies for multidrug-resistant tuberculosis, BMC Medicine,14:89

➢

In combating AMR, pathogen-specific monoclonal antibodies (mAbs) can act as a promising alternative therapy and

it has potential to reduce antibiotic utilization worldwide.

Owing to the recent advancements in molecular biology and antibody therapy, it is possible to generate systematic,

homogenous, fully human, and/or humanized mAbs by using single antigen-specificity to target the desired

pathogen. This process requires only a pathogen antigen (immunogen) and an immunized individual for the

successful development of humanized antibody.

Sortase A (SrtA) inhibitors

➢

SrtA are cysteine trans-peptidases that help in building the cell wall architecture and attachment of cell wall

proteins.These are found ubiquitously in GPB, while genomic data analysis has revealed the presence of sortase

superfamily genes in GNB and archaeal species.

In GPB, cell wall anchored surface protein contains a highly conserved C-terminal LPXTG motif, these cell surface

proteins have shown a role in host-pathogen interactions and can act as a virulence factor, thus can be a potentially

good target for future drug discovery research.

Several studies on SrtA hinted about their essentiality for the integrity of cell surface proteins and for pathogenesis

of S. aureus infections. SrtA activity inhibition might potentially decreases virulence of S. aureus as well as in

Listeria monocytogenes, Streptococcus pneumoniae, Streptococcus suis and Streptococcus mutans.

Different screening strategies have been adopted for screening of novel potential SrtA inhibitors, including

screening of natural compounds, virtual in silico screening and fragment-based lead compound discovery. A

combination of all these screening studies may give better results in exploring repository of uncovered hits for

identifying sortase inhibitors.

➢

Most of the antibiotics target the active site of the enzymes called orthosteric inhibitors. However, the

biological activities of enzymes can also be regulated by allosteric modulators. Allosteric modulators are the

compounds that bind other than at the active site of biocatalyst.

➢

In 2021, Mayland Chang et al. studied a novel allosteric inhibitor ceftaroline that has shown anti-MRSA

activity in screening as well as docking analysis.

Some other example of novel allosteric modulators includes quinazolinones and PBP2a in case of S. aureus

and aminoglycoside-modifying enzymes (AMEs) including acetyltransferase, phosphotransferase and

nucleotidyl transferase.

➢

To date, PBP2a (penicillin-binding protein 2a) is a promising bacterial protein target molecule for the novel

quinazolinone class as well as oxadiazoles class by allosteric inhibition.

T h a n k y o u ！