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Assessment of virulence diversity of methicillin-resistant Staphylococcus aureus strains with a Drosophila melanogaster infection model.

Wu K, Conly J, Surette M, Sibley C, Elsayed S, Zhang K - BMC Microbiol. (2012)

Bottom Line: These results correlate with bacterial virulence in the Caenorhabditis elegans host model as well as human clinical data.Our results demonstrate that the D. melanogaster model is potentially useful for studying S. aureus pathogenicity.Different MRSA strains demonstrated diverse virulence in the D. melanogaster model, which may be the result of differing expression of bacterial virulence factors in vivo.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Pathology & Laboratory Medicine, University of Calgary, 3330 Hospital Drive NW, Calgary, AB, Canada.

ABSTRACT

Background: Staphylococcus aureus strains with distinct genetic backgrounds have shown different virulence in animal models as well as associations with different clinical outcomes, such as causing infection in the hospital or the community. With S. aureus strains carrying diverse genetic backgrounds that have been demonstrated by gene typing and genomic sequences, it is difficult to compare these strains using mammalian models. Invertebrate host models provide a useful alternative approach for studying bacterial pathogenesis in mammals since they have conserved innate immune systems of biological defense. Here, we employed Drosophila melanogaster as a host model for studying the virulence of S. aureus strains.

Results: Community-associated methicillin-resistant S. aureus (CA-MRSA) strains USA300, USA400 and CMRSA2 were more virulent than a hospital-associated (HA)-MRSA strain (CMRSA6) and a colonization strain (M92) in the D. melanogaster model. These results correlate with bacterial virulence in the Caenorhabditis elegans host model as well as human clinical data. Moreover, MRSA killing activities in the D. melanogaster model are associated with bacterial replication within the flies. Different MRSA strains induced similar host responses in D. melanogaster, but demonstrated differential expression of common bacterial virulence factors, which may account for the different killing activities in the model. In addition, hemolysin α, an important virulence factor produced by S. aureus in human infections is postulated to play a role in the fly killing.

Conclusions: Our results demonstrate that the D. melanogaster model is potentially useful for studying S. aureus pathogenicity. Different MRSA strains demonstrated diverse virulence in the D. melanogaster model, which may be the result of differing expression of bacterial virulence factors in vivo.

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MRSA proliferation correlated with fly killing activity. Growth curves of MRSA strains in M9 minimal medium (A) and brain heart infusion (BHI) broth (B) at 25°C for 24 hrs. (C) Growth of MRSA strains within the flies for 24 hrs. A batch of live flies was harvested at 1, 6, 18, and 24 hours post infection and CFU/fly was determined. (D-G) Bacterial counts in different body parts from the flies infected with different MRSA strains at 18 hours post infection: (D) crop; (E) head; (F) wing; (G) leg. The asterisk indicates a statistically significantly difference (p < 0.05) between groups of the high virulence strains and the low virulence strains in bacterial counts in different body parts (Mann–Whitney test). (H-M) Microscopic examination of representative histopathological sections of BHI broth-injected (control) flies (H,K), and M92 (I, L) and USA300-2406 (J, M) infected flies, low (4X) and high magnification (100X) respectively.
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Figure 2: MRSA proliferation correlated with fly killing activity. Growth curves of MRSA strains in M9 minimal medium (A) and brain heart infusion (BHI) broth (B) at 25°C for 24 hrs. (C) Growth of MRSA strains within the flies for 24 hrs. A batch of live flies was harvested at 1, 6, 18, and 24 hours post infection and CFU/fly was determined. (D-G) Bacterial counts in different body parts from the flies infected with different MRSA strains at 18 hours post infection: (D) crop; (E) head; (F) wing; (G) leg. The asterisk indicates a statistically significantly difference (p < 0.05) between groups of the high virulence strains and the low virulence strains in bacterial counts in different body parts (Mann–Whitney test). (H-M) Microscopic examination of representative histopathological sections of BHI broth-injected (control) flies (H,K), and M92 (I, L) and USA300-2406 (J, M) infected flies, low (4X) and high magnification (100X) respectively.

Mentions: The high virulence strains USA300 and USA400 had the highest growth rates in both BHI broth and M9 minimal medium; but CMRSA2 had a lower growth rate and similar virulence to USA300 and USA400 in the fly model (Figure 2A and B), indicating that the growth rate in vitro was not associated with virulence in the fly model. On the other hand, in vivo results indicated that the high virulence strains had a higher growth rate than the low virulence strains in vivo. At 1 hour post infection, similar bacterial counts (0.43 × 104 to 0.83 × 104 CFU/fly) were observed for all MRSA strains (Figure 2C). The bacterial counts per fly increased by time indicating that bacterial replication was occurring and 1.8 × 104 - 4.2 × 104 CFU/fly were observed for all strains at 6 hours. Following the 6 hour mark, the high virulence strains, USA300, USA400 and CMRSA2, grew exponentially and the viable bacterial counts were 0.77 × 108-1.7 × 108 CFU/fly by 18 hours. The low virulence strains grew more slowly and by 18 hours the viable bacterial counts were 0.72 × 106 CFU/fly for CMRSA6 and 1.4 × 106 CFU/fly for M92. A significant difference was observed between the high virulence strains and the low virulence strains (p=0.003). At 24 hours post infection with the high virulence strains, dead flies were excluded from the experiment. With the surviving flies, the viable bacterial concentration per fly was approximately 107 CFU/fly for USA300 and CMRSA2 infected flies, and 108 CFU/fly for USA400. With CMRSA6 and M92 infected flies, the bacterial counts were about 3.0 × 106 CFU/fly at 24 hours.


Assessment of virulence diversity of methicillin-resistant Staphylococcus aureus strains with a Drosophila melanogaster infection model.

Wu K, Conly J, Surette M, Sibley C, Elsayed S, Zhang K - BMC Microbiol. (2012)

MRSA proliferation correlated with fly killing activity. Growth curves of MRSA strains in M9 minimal medium (A) and brain heart infusion (BHI) broth (B) at 25°C for 24 hrs. (C) Growth of MRSA strains within the flies for 24 hrs. A batch of live flies was harvested at 1, 6, 18, and 24 hours post infection and CFU/fly was determined. (D-G) Bacterial counts in different body parts from the flies infected with different MRSA strains at 18 hours post infection: (D) crop; (E) head; (F) wing; (G) leg. The asterisk indicates a statistically significantly difference (p < 0.05) between groups of the high virulence strains and the low virulence strains in bacterial counts in different body parts (Mann–Whitney test). (H-M) Microscopic examination of representative histopathological sections of BHI broth-injected (control) flies (H,K), and M92 (I, L) and USA300-2406 (J, M) infected flies, low (4X) and high magnification (100X) respectively.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3539928&req=5

Figure 2: MRSA proliferation correlated with fly killing activity. Growth curves of MRSA strains in M9 minimal medium (A) and brain heart infusion (BHI) broth (B) at 25°C for 24 hrs. (C) Growth of MRSA strains within the flies for 24 hrs. A batch of live flies was harvested at 1, 6, 18, and 24 hours post infection and CFU/fly was determined. (D-G) Bacterial counts in different body parts from the flies infected with different MRSA strains at 18 hours post infection: (D) crop; (E) head; (F) wing; (G) leg. The asterisk indicates a statistically significantly difference (p < 0.05) between groups of the high virulence strains and the low virulence strains in bacterial counts in different body parts (Mann–Whitney test). (H-M) Microscopic examination of representative histopathological sections of BHI broth-injected (control) flies (H,K), and M92 (I, L) and USA300-2406 (J, M) infected flies, low (4X) and high magnification (100X) respectively.
Mentions: The high virulence strains USA300 and USA400 had the highest growth rates in both BHI broth and M9 minimal medium; but CMRSA2 had a lower growth rate and similar virulence to USA300 and USA400 in the fly model (Figure 2A and B), indicating that the growth rate in vitro was not associated with virulence in the fly model. On the other hand, in vivo results indicated that the high virulence strains had a higher growth rate than the low virulence strains in vivo. At 1 hour post infection, similar bacterial counts (0.43 × 104 to 0.83 × 104 CFU/fly) were observed for all MRSA strains (Figure 2C). The bacterial counts per fly increased by time indicating that bacterial replication was occurring and 1.8 × 104 - 4.2 × 104 CFU/fly were observed for all strains at 6 hours. Following the 6 hour mark, the high virulence strains, USA300, USA400 and CMRSA2, grew exponentially and the viable bacterial counts were 0.77 × 108-1.7 × 108 CFU/fly by 18 hours. The low virulence strains grew more slowly and by 18 hours the viable bacterial counts were 0.72 × 106 CFU/fly for CMRSA6 and 1.4 × 106 CFU/fly for M92. A significant difference was observed between the high virulence strains and the low virulence strains (p=0.003). At 24 hours post infection with the high virulence strains, dead flies were excluded from the experiment. With the surviving flies, the viable bacterial concentration per fly was approximately 107 CFU/fly for USA300 and CMRSA2 infected flies, and 108 CFU/fly for USA400. With CMRSA6 and M92 infected flies, the bacterial counts were about 3.0 × 106 CFU/fly at 24 hours.

Bottom Line: These results correlate with bacterial virulence in the Caenorhabditis elegans host model as well as human clinical data.Our results demonstrate that the D. melanogaster model is potentially useful for studying S. aureus pathogenicity.Different MRSA strains demonstrated diverse virulence in the D. melanogaster model, which may be the result of differing expression of bacterial virulence factors in vivo.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Pathology & Laboratory Medicine, University of Calgary, 3330 Hospital Drive NW, Calgary, AB, Canada.

ABSTRACT

Background: Staphylococcus aureus strains with distinct genetic backgrounds have shown different virulence in animal models as well as associations with different clinical outcomes, such as causing infection in the hospital or the community. With S. aureus strains carrying diverse genetic backgrounds that have been demonstrated by gene typing and genomic sequences, it is difficult to compare these strains using mammalian models. Invertebrate host models provide a useful alternative approach for studying bacterial pathogenesis in mammals since they have conserved innate immune systems of biological defense. Here, we employed Drosophila melanogaster as a host model for studying the virulence of S. aureus strains.

Results: Community-associated methicillin-resistant S. aureus (CA-MRSA) strains USA300, USA400 and CMRSA2 were more virulent than a hospital-associated (HA)-MRSA strain (CMRSA6) and a colonization strain (M92) in the D. melanogaster model. These results correlate with bacterial virulence in the Caenorhabditis elegans host model as well as human clinical data. Moreover, MRSA killing activities in the D. melanogaster model are associated with bacterial replication within the flies. Different MRSA strains induced similar host responses in D. melanogaster, but demonstrated differential expression of common bacterial virulence factors, which may account for the different killing activities in the model. In addition, hemolysin α, an important virulence factor produced by S. aureus in human infections is postulated to play a role in the fly killing.

Conclusions: Our results demonstrate that the D. melanogaster model is potentially useful for studying S. aureus pathogenicity. Different MRSA strains demonstrated diverse virulence in the D. melanogaster model, which may be the result of differing expression of bacterial virulence factors in vivo.

Show MeSH
Related in: MedlinePlus