Cross no. | Flask set | Parental MOI | No. of P-0 flasks with IFU^{b} | Fisher's exact P value | ||
---|---|---|---|---|---|---|

First mutant | Second mutant | + | − | |||

1 (OFX^{r}-1 × LIN^{r}-1) | A | 5.0 | 0 | 10 | ||

B | 5.0 | 2 | 8 | |||

C | 5.0 | 5.0 | 10 | 0 | <10^{−3}
| |

D | 5.0 | 0.5 | 10 | 0 | <10^{−3}
| |

2 (OFX^{r}-2 × LIN^{r}-1) | A | 5.0 | 0 | 10 | ||

B | 5.0 | 2 | 8 | |||

C | 5.0 | 5.0 | 10 | 0 | <10^{−3}
| |

3 (OFX^{r}-1 × RIF^{r}-1) | A | 6.8 | 3 | 7 | ||

B | 5.1 | 5 | 5 | |||

C | 5.1 | 6.8 | 10 | 0 | <10^{−2}
| |

4 (OFX^{r}-1 × TMP^{r}-1) | A | 5.0 | 0 | 10 | ||

B | 5.0 | 1 | 9 | |||

C | 5.0 | 5.0 | 10 | 0 | <10^{−3}
| |

5 (OFX^{r}-1 × LIN^{r}-1) | I | 5.0 | 10^{−1}
| 10 | 0 | NA |

II | 5.0 | 10^{−2}
| 10 | 0 | NA | |

III | 5.0 | 10^{−3}
| 10 | 0 | NA | |

IV | 5.0 | 10^{−4}
| 10 | 0 | NA | |

V | 5.0 | 10^{−5}
| 10 | 0 | NA |

↵

*a*The numbers of infected P-0 flasks that yielded IFU simultaneously resistant to two selection antibiotics are shown. In crosses no. 1 to no. 4, P-0 HeLa flasks were infected with one kind of mutant (set A), the other kind of mutant (set B), or both (set C and others as specified) and were then passaged with selection as described in the text. P-4 flasks either lacked doubly-resistant inclusions (−) or had numerous doubly-resistant inclusions (+) (Fig. 3). The presence of abundant inclusions in P-4 flasks identified antecedent P-0 flasks that contained spontaneous mutants (sets A and B) or LGT recombinants (sets C and, in cross no. 1, D). Uniparental infections were not necessary in cross no. 5 and were omitted. Instead, 10-flask sets (sets I to V) of HeLa flasks were simultaneously infected with both parental strains at the MOI shown (first and second mutants refer to the mutants in the order listed for the cross in the far-left column). Each*P*value is the probability that the difference between uniparental and biparental infections in the numbers of flasks marked + and flasks marked − resulted from chance, alone, according to Fisher's exact test. NA, not applicable.↵

*b*For crosses no. 1, 2, and 5, OFX^{r}LIN^{r}IFU; for cross no. 3, OFX^{r}RIF^{r}IFU; and for cross no. 4, OFX^{r}TMP^{r}IFU.