Table 2 Relationships between the CRISPR-Cas system and bacterial virulence

Enhanced virulence

CRISPR-Cas system prevents bacteria from forming strong virulent strains with a capsule

CRISPR1 locus of S. pyogenes

S. pneumoniae

[62]

Cas9 mediates the immune escape of TLR2, which increases the toxicity of bacteria

CRISPR-Cas9 system

F. novicida

[197]

The deletion of CRISPR promotes the insertion of virulence genes and enhances virulence

CRISPRs of V. parahaemolyticus

V. parahaemolyticus

[127]

There is significant correlation between the virulence factor tdh gene and the CRISPR-Cas system

Type II CRISPR-Cas system (subtype I-F)

V. parahaemolyticus

[126]

Lack of CRISPR promotes the insertion of prophages from HGT

CRISPRs of V. parahaemolyticus

V. parahaemolyticus

[128]

CRISPR systems resist phage invasion, regulate bacterial virulence and biofilm formation, and promote the evolution of L. monocytogenes towards high virulence

RliB-CRISPR, CRISPR I-B and CRISPR II-A of L. monocytogenes

L. monocytogenes

[198]

RliB-CRISPR forms a stem-ring structure and regulates the virulence of bacteria

RliB-CRISPR

L. monocytogenes

[199]

There is no correlation between the I-E CRISPR-Cas system and virulence genes, but the total number of spacer regions is negatively correlated with potential pathogenicity

CRISPR1- CRISPR4 (subtype I-E)

E. coli

[122]

There is a negative correlation between the number of I-E CRISPR loci and pathogenic traits. Higher numbers of virulence factors result in lower repeat contents

CRISPR2 (subtype I-E)

E. coli

[123]

The absence or presence of I-F system in bacteria may affect the distribution of virulence or ARGs

CRISPR-Cas system (subtype I-F)

E. coli

[125]

The CRISPR system prevents the acquisition of some virulence factors, which is negatively correlated with the existence of some virulence factors

CRISPR1-cas, orphan CRISPR2, and CRISPR3-cas

E. faecalis

[200]

Cas3 gene deletion mutant strains have increased virulence

Type I CRISPR-Cas3 system

P. gingivali

[131]

Phage resistance may be related to low virulence, which makes non-phage-resistant strains more virulent

CRISPR-Cas system (cas1, cas3-cas2, and cas6)

A.baumannii

[130]

The active CRISPR system of B. thuringiensis strains hinders HGT, including the transfer of virulence genes. Therefore, they have lower virulence than strains without an active CRISPR system

CRISPR-Cas system (subtypes I-C) of B. cereus strain

B. cereus

[129]

Reduced virulence

The expression level of several virulence genes in Cas3-deficient S. mutants is decreased

CRISPR1 system (type II-A) and CRISPR2 system (type I-C)

S. mutans

[201]

The deletion of csn2 in S. mutants has multiple effects on pathogen virulence through gene expression changes

CRISPR-Cas9 system (csn2 gene)

S. mutans

[202]

Inactivation of the csn1 gene reduces the virulence of cst-II positive C. jejuni isolates

Type II CRISPR-Cas system

C. jejuni

[135]

The virulence, adhesion ability, and survival ability of Δcas9 mutant strains are lower than those of wild-type strains

Type II CRISPR-Cas9 system

C. jejuni

[136]

PA14 changes the virulence of bacteria by targeting and inhibiting LasR, and the bacteria has the ability to escape host defences

Types I-F CRISPR-Cas system of PA14

P. aeruginosa

[132]

The presence of an active CRISPR-Cas system is associated with increased virulence

CRISPR-Cas systems (subtypes I-F, I-E, I-C)

P. aeruginosa

[203]

P. aeruginosa maintains its CRISPR Cas system by inhibiting its toxicity

CRISPR-Cas system of PA14

P. aeruginosa

[204]

The ΔCas9 mutant strains constructed with high-virulence clinical strains have low virulence, invasiveness, and adhesion ability

Type II CRISPR-Cas9 system

Group B Streptococcus

[205]

Cas3 is involved in Salmonella biofilm formation and bacterial invasion, and it activates virulence

Type I CRISPR-Cas3 system (subtype I-E)

Salmonella

[133]

Strains with the I-E* CRISPR-Cas system have higher virulence

CRISPR-Cas systems (I-E and I-E*)

K. pneumoniae

[206]