Bacterial Information	Value
Taxonomy level	Species
NCBI Taxonomy ID	287
Phylum	Proteobacteria
Family	Pseudomonadaceae
Genus	Pseudomonas
Gram stain	Gram-negative
Oxygen requirements	Aerobic, facultatively-anaerobic
Spore-forming	No
Motile	Yes
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• Pseudomonas aeruginosa
  • Table of Contents
  • Overview
    • Pathogenesis
      • Adhesins
      • Secretion systems
      • Secreted toxins and exoenzymes
      • Injected toxins
      • Mucoid phenotype conversion
      • Antibiotic resistance
      • Quorum-sensing regulation
  • Biological information
    • Detection
    • Airway inflammation
      • Adhesin molecules stimulate IL-8 secretion
      • Increases in VEGF

Pseudomonas aeruginosa is a Gram-negative, rod-shaped bacterium that does not form spores. It is motile via a single polar flagellum (swimming/swarming motility) and also its type IV pili (twitching motility). P. aeruginosa grows well at 25 – 37°C, and its ability to grow at 42°C distinguishes it from other Pseudomonas species.

P. aeruginosa is a ubiqitous organism with the ability to survive in a multitude of environmental conditions. It can be found in soil, plants, and animals including humans.

The majority of strains produce pigments including pyocyanin (blue-green), pyoverdine (yellow-green and fluorescent), and pyorubin (red-brown). Pyocyanin contributes to persistence of P. aeruginosa in the lungs of cystic fibrosis (CF) patients, and may also interfere with mammalian cell function, including cellular respiration, ciliary beating, epidermal cell growth, calcium homeostasis, and prostacyclin release from endothelial cells (although the exact mechanisms are poorly understood)(Caldwell 2009).

The genome of P. aeruginosa is relatively large at 5.5 – 7 Mbp, with a high G+C content of 65 – 67 mol%. Its large genome contributes to its considerable metabolic versatility. About 8% of its genome encodes for regulatory genes, enabling adaptation to diverse and complex environmental conditions.

• Type IV pili:
  • The type IV pili of P. aeruginosa are flexible and retractable filaments with multiple function including surface motility, biofilm and microcolony formation, adhesion, immune evasion, transformation of DNA, cell signalling, and phage attachment; their disruption reduces bacterial virulence (Hahn 1997).
  • About 90% of adhesion capability to host epithelial cells is dependent on these type IV pili.
• Flagella:
  • P. aeruginosa possesses a single polar flagellum composed of flagellin (encoded by FliC - there are many other proteins associated with assembly and function). Flagella are required for multiple purposes including adhesion, motility, and biofilm formation – their loss reduces pathogenicity and host epithelial cell invasion (Dasgupta 2003).
  • Host-derived mucins, both secreted and cell-associated, have been shown to bind to P. aeruginosa flagella.
  • Flagellin can also bind to asialo ganglio-N-tetraosylceramide (GM1) and TLR5, thereby activating host innate immune responses (Hayashi 2001).
  • At the basolateral surface, flagella can mediate adhesion to the heparan sulfate chains of heparan sulfate proteoglycans on host polarised epithelial cells, resulting in activation of epithelial growth factor receptor (EGFR) and downstream PI3K/Akt signalling, and subsequent bacterial invasion (Bucior 2012).
• Core oligosaccharide of LPS:
  • The core oligosaccharide of P. aeruginosa LPS has been shown to be a ligand for the cystic fibrosis transmembrane regulator (CFTR) receptor, and mediates the internalisation of the bacteria by host epithelial cells.
    • The CFTR defect in CF disease results in increased mucin production and poor mucociliary clearance that favours P. aeruginosa survival and propagation.
    • P. aeruginosa LPS outer core specifically recognises CFTR amino acid sequence 108 – 117 (located in the first extracellular domain). This interaction can activate the formation of lipid rafts for subsequent bacterial internalisation.

• Type I secretion system (T1SS):
  • The T1SS is composed of an outer membrane protein and an ABC transporter. Its secreted proteins have a C-terminal secretion signal.
  • T1SS in P. aeruginosa include:
    • The Apr system involved in secretion of the alkaline protease AprA.
    • The HasAp system involved in iron utilisation – HasAp is a haemophore (binds haem from haemoglobin) and is considered crucial for early infection (Delepelaire 2004).
• Type II secretion system (T2SS):
  • The T2SS uses a two-step process to deliver extracellular proteins:
    • Firstly, a Sec- or Tet-dependent delivery system transports proteins from the cytosol into the periplasm.
    • Secondly, the T2SS complex-mediated secretion delivers the proteins to the extracellular space.
    • These steps are accomplished by the Xcp (extracellular protein) and Hxc (homologous to the extracellular protein) systems in P. aeruginosa.
  • Proteins such as LasA, LasB, PrpL, exotoxin A, and phospholipase C are secreted via T2SS.
• Type III secretion system (T3SS):
  • T3SS can inject toxic protein directly into the cytosol of target host cells using a needle-like appendage.
  • ExoS, ExoT, ExoU, and ExoY are some of the effector proteins injected by the T3SS.
• Type V secretion system (T5SS):
  • The T5SS involves a two-step process like T2SS. The first step is Sec-dependent translocation to the periplasm, and the second step involves either autotranslocation (via the C-terminal signal) or the use of a helper protein.
  • Secreted proteins include EstA, LepA, and LepB.
• Type VI secretion system (T6SS):
  • T6SS are needle-like complexes like T3SS, and those identified in P. aeruginosa include HSI-I, HSI-II, and HSI-III.
    • HSI-I is a dynamic contractile phage tail-like structure that protrudes from the cytosol to the bacterial surface (Basler 2012).
  • Effector proteins for HSI-I include Tse1, Tse2, and Tse3, which target other bacterial cells (likely for competitive advantage)(Russell 2011).
  • HSI-II has been shown to enhance bacterial internalistion into host epithelial cells (Sana 2012), and both HSI-II and HSI-III have roles in mediating pathogenesis (Lesic 2009).

• Exotoxin A (PE):
  • One of the most important virulence factors for P. aeruginosa, it is secreted by T2SS and binds to receptors on host cells. Its ADP-ribosyltransferase activity modifies and inactivates elongation factor 2 (eEF-2), resulting in inhibition of protein synthesis and subsequent cell death.
  • It is activated by eukaryotic protease furin (necessary for its toxic effect), and contains a C-terminal endoplasmic reticulum retention sequence.
• Elastases:
  • LasA and LasB, secreted via T2SS, have elastolytic activity. These elastases are able to degrade the elastin in host connective tissue (an element normally robust against most proteases).
• Alkaline protease (AP):
  • A Zn²⁺-metalloprotease secreted via T1SS, alkaline protease can activate the epithelial sodium channel (ENaC) in CF disease. It can also degrade human IFN-γ and inhibit opsonised zymosan-stimulated neutrophil oxygen consumption.
• Phospholipase C (PLC):
  • Pseudomonas PLCs are secreted via T2SS, and be either haemolytic (PlcH) or non-haemolytic (PlcN) in nature.
    • Host PLCs catalyse the synthesis of diacylglycerol (DAG) from the hydrolysis of phosphatidylinositol or phosphatidylcholine – DAG metabolism is important in various signalling pathways including apoptosis, oncogenesis, and inflammation.
  • PlcH can degrade phosphatidylcholine and sphingomyelin in eukaryotic cell membranes, and can degrade lung surfactant (Wargo 2009).
• Rhamnolipids:
  • These are biosurfactants and surface-active amphipathic molecules, and are produced by rlhA, rhlB, and rlhC, under the regulation of a quorum sensing system.
  • Initially they were identified as heat-stable haemolysins, able to affect macrophage function, mucociliary transport and ciliary beating (Johnson 1980). They also have roles in modulating swarming motility and biofilm architecture. In addition to the finding they are required for biofilm detachment, iron-limited conditions have been shown to upregulate rhamnolipid production and twitching motility (Glick 2010).

Expression of T3SS genes is tightly controlled by multiple regulators. Environmental stimuli such as low calcium or host cell contact can downregulate the transcriptional repressors.

• ExoS and ExoT:
  • These possess ADP-ribosyltransferase (ADPRT) and GTPase-activity protein (GAP) activity.
    • ExoS preferentially ADP-ribosylates several of the Ras family of GTP-binding proteins required for intracellular vesicle transport, cell proliferation, and differentiation. This ADPRT activity causes programmed cell death in various types of cultured cells – mutants are incapable of inducing apoptosis in vitro (Kaufman 2000).
• ExoU:
  • This toxin has lipase activity.
• ExoY:
  • This toxin has adenylate cyclase activity.

Overproduction of alginate, a polymer of D-mannuronate and L-glucuronate, enables conversion toward a mucoid phenotype in vivo, allowing persistance in the airways of CF patients – it is considered an indicator of poor prognosis.

• Mucoid phenotype P. aeruginosa biofilms are protected from environmental stresses. Host complement activation is inhibited, phagocytosis by neutrophils and macrophages is decreased, and bacteria are protected from free radicals produced by host immune responses.
• The mucoid phenotype is associated with lower toxicity, with many invasive factors being downregulated, e.g. T3SS, LasA, LasB, and flagellar motility.

P. aeruginosa are highly drug resistant, both by intrinsic resistance (chromosomally-encoded) and acquired resistance (i.e. from mobile genetic elements).

• ampC-mediated resistance to β-lactams:
  • All four β-lactamase classes have been identified in P. aeruginosa.
    • Class C (cephalosporinase, encoded by ampC) and class D (oxacillanase, encoded by poxB) are encoded chromosomally.
    • When AmpC is sufficiently increased, P. aeruginosa is resistant to nearly all β-lactams, with the exception of the carbapenams (Sanders 1986).
    • Carbapenamase-mediated resistance in P. aeruginosa is an emerging concern however, with 10-30% of isolates in the United States displaying resistance (Tenover 2022). This resistance is conferred by class A β-lactamases KPC and GES and metallo-β-lactamases IMP, NDM, SPM, and VIM, alongside class D OXA-48 enzymes.
• Aminoglycoside resistance:
  • Resistance is conferred by acquired or chromosomal-encoded aminoglycoside-modifying enzymes (AMEs); these are typically common in P. aeruginosa, but interestingly uncommon in CF isolates (Islam 2008).
• Efflux-mediated resistance:
  • These mechanisms work to reduce antibiotic concentration in the bacterial cytoplasm; P. aeruginosa employs both membrane impermeability and drug efflux pumps to achieve this.
    • The outer membrane of all Gram-negative bacteria, P. aeruginosa included, naturally prevents influx of large, hydrophobic molecules.
    • Conversely, hydrophilic molecules can diffuse into cells via porins. The major porin on P. aeruginosa cells is OprF. Loss of OprF has not been linked with increased resistance, however loss of another porin OprD has been associated with carbapenam resistance in some clinical isolates (Li 2012).
  • Members of the resistance-nodulation-division (RND) family are the largest contributors to P. aeruginosa drug resistance, with 12 RND systems encoded. These consist of a periplasmic membrane fusion protein (MFP), an outer-membrane factor (OMF), and a cytoplasmic membrane (RND) transporter.
    • The MexAB-OprM efflux pump exports β-lactams (and β-lactamase inhibitors), tetracyclines, macrolides, fluoroquinolones, chloramphenicol, trimethoprim, and novobiocin. It is constitutively expressed in wild-type P. aeruginosa and is involved in intrinsic resistance.
    • MexCD-OprJ and MexEF-OprN have similarity to MexAB-OprM, but have varying preferences for export activity.
    • The MexXY efflux pump exports specific β-lactams (e.g. cefepime), aminoglycosides, fluoroquinolones, erythromycin, chloramphenicol, and tetracycline.

P. aeruginosa has three quorum-sensing (QS) systems. There are two LuxI/LuxR-type systems (las and rhl) dependent on N-acyl homoserine lactones (AHLs), and a Pseudomonas quinolone signalling system (PQS). These QS systems are arranged hierarchically: the las system positively regulates the rhl and pqs systems, the rhl system negatively regulates the pqs system, and the pqs system autoinduces itself while simultaneously activating rhl expression. There are many additional regulators at the transcriptional, translational, and post-translational level (Wu 2015). There are many traits controlled by the three QS systems, including virulence factor regulation, biofilm maturation, and motility.

Pseudomonas aeruginosa is a relatively ubiquitous microorganism in the environment, however it is also an adaptable pathogen in mammals and plants for example. It is usually associated with infection in humans, however low numbers may be detected outside of a disease-context.

In humans, it has been detected in:

• Airways: it is typically found in cystic fibrosis patients, and can be detected in pneumonia. An underlying condition such as cystic fibrosis or bronchiectasis predisposes patients to respiratory infections.
• Urine: it can be present in cases of urosepsis.
• Blood: Pseudomonas infections can lead to bacteraemia.
• Skin: outside of environmental exposure, it can be found in conditions such as folliculitis, green nail, or hot foot syndromes associated with recreational exposure to water soures such as hot tubs or swimming pools.
• Burn and wound sites: can cause infection, including secondary infection of burn sites.
• Joints: it can be present in some cases of septic arthritis, especially in smaller joints.
• Ears: it can cause inflammation and swelling of the outer ear canal in cases of otitis externa (also known as Swimmer's ear). Particularly in diabetic or immune compromised individuals, this may manifest as severe, potentially life-threatening malignant otitis externa.

Binding of P. aeruginosa surface molecules such as pilin and flagellin to their receptors on respiratory epithelial cells results in the production of the neutrophil chemoattractant IL-8 (DiMango 1995). Non-piliated mutants show decreased binding to respiratory epithelial cells, and stimulate far less IL-8 production.

Related to this, P. aeruginosa type IV pilin binds to asialylated glycolipids which are increased in cystic fibrosis (CF) due to limited acidification within the trans Golgi and subsequent diminished sialylation of cell glycoconjugates. As such, Pseudmonas pilin binds to the increased asialoGM1 found on CF epithelial cells, but not the predominantly sialylated form found on normal cells (Saiman 1993)(Barasch 1991).

P. aeruginosa supernatant in vitro or persistent PAO1 infection in vivo results in marked increases in vascular endothelial growth factor (VEGF) production by airway epithelial cells and stimultion of peribronchial angiogenesis (Martin 2011). Addition of epidermal growth factor receptor (EGFR) inhibitors reduced VEGF synthesis and reduced the increase in small peribronchial vessels in female C57/BL6 mice.

The correct balance here is essential however. It has also been observed that VEGF expression by type II alveolar epithelial cells is necessary in providing adequate host defence in response to bacterial infection. In VEGF-deficient lungs, increased levels of PAO1 and pro-inflammatory cytokines (TNFα and IL-6) were detected 24 hours after bacterial administration compared to control lungs (Breen 2013). Lipid metabolism in the type II cells was somewhat altered, with decreases in sphingomylein content, choline phosphate cytidylyltransferase (CCT) mRNA, and surfactant protein D.